It became apparent that this influenza was a first-rate killer.
In the last week of October, 1918, 2,700 Americans died “over there” in battle against the kaiser’s army. The same week 21,000 Americans died of influenza in the United States.
The epidemic was by then raging over most of the populated globe, yet where the outbreak had begun was unclear. Some medical men blamed Chinese workers brought to France to dig trenches. A Spanish medical commission proved, at least to its own satisfaction, that the sickness had originated in Russian Turkestan. The Russians, and most of the world, attributed it to Spain. Months before, an influenza epidemic had swept that country like a tidal wave, afflicting eight million people. This earlier outbreak had been mild, however, with few if any deaths directly resulting from it. But, fairly or not, the deadly contagion now gripping the world became known as the Spanish influenza.
To the extent that the origins of this medical forest fire could be traced anywhere, the best evidence pointed to the United States. The previous March a severe dust storm had obscured the sun at Fort Riley, Kansas. Some nine thousand tons of manure were burned every month at this prairie cavalry post, continuously mantling the area in a malodorous haze. The storm winds had whipped up a stinging blizzard of dust and smoke that sent soldiers stumbling, coughing, and choking to the refuge of their barracks.
Two days after the storm had ceased, an army cook named Albert Gitchell reported to the post hospital, complaining of fever, sore throat, and various aches and pains. Minutes later another soldier checked in with the same symptoms. The count had jumped to 107 similarly afflicted patients by midday; by week’s end, 522; and before the sickness ran its course five weeks later, 1,127 men had been stricken. Forty-six of them died. The base surgeon diagnosed the sickness as influenza, although pneumonia was blamed for the deaths. In May of 1918 the Army’s 89th and 92nd divisions finished their training at Fort Riley and sailed for France.
Soon after the 92nd Division disembarked at Brest and Saint-Nazaire, French poilus began to fall ill with influenza. British soldiers in France carried the disease back to England. Influenza spread through the Royal Navy like flames on an oil slick. Over 10,000 British tars were laid low, confining the fleet to port. The disease rolled across France and into Germany, where eventually 160,000 Berliners came down with the flu. It erupted halfway around the world, sweeping across China, India, and most of Asia—whether carried from Europe or appearing independently no one knew.
On August 12, four months after the men from Fort Riley had sailed for France, a Norwegian liner, the Bergensfjord , picked its way slowly through the fog of New York Harbor and tied up at the army base in Brooklyn. Passengers streamed down the gangway as quickly as they could escape the ship. The voyage had been a nightmare. Four of their fellow passengers had died on board and had been given a speedy burial at sea. The dead were among 100 passengers on the Bergensfjord stricken with influenza.
Bells clanged along the dock as ambulances rushed semiconscious patients to Brooklyn’s Norwegian Hospital. There, hours later, the disease claimed its fifth victim from the Bergensfjord , a Mrs. Olsen, the first person to die in America from the latest recurrence of influenza. The plague had apparently come full circle.
An influenza epidemic was hardly a medical novelty. In 412 B.C. Hippocrates recorded an outbreak, closely resembling influenza, that had wiped out an Athenian army. Since then fairly serious outbreaks apparently occurred approximately every hundred years. Influenza had surfaced in American history at Valley Forge (George Washington came down with the flu during the winter of 1779) and again during the Civil War. The name of the disease, derived from the Italian word for “influence,” suggests its intangible and mysterious quality; and since the disease is caused by a virus, it is still mysterious.
Though influenza had intermittently struck hard in the past, its behavior in the years prior to 1918 had been relatively tame. As a cause of death influenza ranked about tenth, well behind heart disease, pneumonia, tuberculosis, and cancer. Yet by the fall of 1918 it had become apparent that this outbreak was a first-rate killer. On October 1, 202 persons died in Boston from influenza. On October 6 Philadelphia posted a grim milestone, 289 flu deaths in twenty-four hours—the highest mortality that had ever been recorded in that city’s history. At an emergency meeting in New York City the Henry Street Settlement House reported that of 170 women on the settlement house’s visiting-nurse staff, 31 had already-fallen prey to the disease.
The swift spread of a highly contagious infection was to be expected in densely populated cities, but influenza made inexplicable leaps. A public-health nurse told of traveling deep into Michigan lumber-camp country on a railroad handcar, the only available conveyance, to help isolated knots of people found burning with fever, huddled in rude log cabins. In Oregon medical authorities were at a loss to explain the appearance of the disease among families of sheepherders living in remote cabins separated from each other by miles of open country.
In October Congress approved a special $1-million fund to enable the United States Public Health Service to recruit physicians and nurses for the emergency. Dr. Rupert Blue, surgeon general of the United States, set out to hire 1,085 physicians and 703 nurses with the new funds. The bottom of the barrel was scraped; some recruits came from homes for the aged, and one doctor was a drug addict. But Dr. Blue managed to find 1,000 recruits who met the minimal standard: they were all licensed to practice medicine.
In any case the influenza epidemic was proving a humbling experience for the medical profession. This was the generation that had clinically developed the germ theory of disease and had discovered vaccines or devised preventive defenses against typhoid fever, diphtheria, tetanus, meningitis, tuberculosis, malaria, and yellow fever. But influenza yielded to no known medication. Officials had to fall back on the most rudimentary public- and personal-health measures.
Early in October, Louis Brownlow, the only health commissioner not down with influenza in Washington, D.C., closed the schools, theatres, and barrooms and suspended all public gatherings. At the National Burlesque “Flo-Flo” and her “Perfect 36” chorus rang down a final curtain on the Frolics of the Night , and the girls admirably responded to an appeal for volunteers to help out at a Washington emergency hospital. Woodrow Wilson had to forgo a favored diversion from the burdens of a wartime Presidency when B. F. Keith’s vaudeville theatre closed. A week or so earlier Boston had canceled its Liberty Bond parades and sports events, closed its churches, and put the stock exchange on half days.
Other communities resisted shutting down profitable enterprises. In Kansas City, Missouri, “Boss” Tom Pendergast refused to close the places of amusement, including saloons. Some 1,800 persons would eventually die of influenza in his town. The health commissioner of New York City, Dr. Royal Copeland, a politically ambitious eye specialist with no public-health background, heard the arguments of theatre people that a shutdown of their enterprises would only provoke panic. Copeland noted that only two per cent of the city’s population was sick with the disease and left the theatres open. On the day of his decision 354 people died in New York of influenza and related pneumonia. Still, no one could say with any certainty whether one official’s action or another’s inaction exerted a particle of influence over the progress of the epidemic.
People who ventured out faced a flurry of new civil ordinances. San Francisco was one of many cities that required everyone to wear face masks in public. In New York City huge signs appeared, warning that public coughing and sneezing without a handkerchief was unlawful and threatening violators with stiff fines and jail sentences. Within a few days dutiful policemen had hauled 500 hapless sneezers and coughers into court. Chicago had a similar ordinance, and lawmen there had plenty of time to enforce this new regulation; the epidemic had reduced the crime rate in their city for the month of October by 43 per cent. The Colgate company did its part by placing ads that described the twelve rules for preventing Spanish influenza. Among the recommendations: chew food carefully, avoid tight clothes, tight shoes, and tight gloves, and breathe as much fresh air as possible.
But the epidemic raged on. New York City recorded 3,077 new cases of influenza in one twenty-four-hour period. One in ten died. In Washington a public-health doctor noted that the only way he could assure room in his emergency hospital in an F Street storefront was to station undertakers at the door to remove the dead promptly. Health Commissioner Brownlow, at home with his sick wife, received a desperate call from a young woman. Of her three roommates two were already dead and the third was dying. Brownlow called the police and asked them to investigate. He soon received a sergeant’s terse report: “Four girls dead” at that address.
Philadelphia was staggered. The early record of 289 deaths in one day was easily surpassed. On October 10, while firemen hosed down the streets all day and people faithfully wore their face masks outside, 528 Philadelphians perished from influenza. The fury of this mortality rate can perhaps be better imagined in terms of 528 Philadelphians dying in a single day in traffic accidents or in a fire rather than, prosaically, in bed.
All places of public assembly in the city were closed, and Philadelphia began to resemble the London described by an English official during the great bubonic plague of 250 years before: ”… the streete thin of people, the shops shut up, and all in mournful silence, as not knowing whose turne might be next.” Even the death carts of London’s Great Plague were recalled in Philadelphia. The Reverend Dr. Joseph Corrigan, director of Catholic Charities, assembled a convoy of six horse-drawn wagons and a truck that scoured the city’s back streets and alleyways in search of abandoned victims. Forcing open doors in cheap tenements and rundown rooming houses, the priest and his helpers gathered up some 200 bodies in twenty-four hours. They deposited their grim harvest in a morgue built to accommodate 36 dead, where conditions soon became so offensive that veteran embalmers recoiled and refused to enter.
Responding to this kind of congestion, the J. F. Brill Company, Philadelphia streetcar manufacturers, temporarily turned its woodworking shop over to the construction of coffins to ease the shortage. In Baltimore the mayor, fearful of contagion from unburied influenza victims, urged undertakers to speed up their work, but the morticians complained that they were hampered by the insistence of families on expensive coffins, which were sold out. The acting health commissioner of Buffalo, New York, announced that his city would begin the manufacture of coffins. “They will not be $1,000 caskets or even $100 caskets,” he said. “They will be plain, with plain handles, and respectable. … The casket business,” he added irritably, “is a worse trust than oil.” In Pittsburgh stacked coffins lined the street for a city block. They were all used. A health official in Washington, B.C., had been tipped off by a railroad employee that two carloads of coffins due for delivery to Pittsburgh were in the Washington freight yards. He hijacked the entire shipment and assigned police to guard the coffins.
What was this unseen destroyer that was taking life in unprecedented numbers? What caused it? How did it travel? Why did it kill?
Dr. Simon Flexner, brilliant head of the department of pathology and bacteriology at the Rockefeller Institute, directed one of his key researchers to devote all her attention to detecting the cause of the disease. Despite a valiant effort, she and her colleagues were unable to isolate or identify the agent, concluding only that the epidemic was indeed influenza, caused by some unknown germ.
Meanwhile less scientific sources proposed other explanations. According to one theory poison gases used in the war, air charged with carbon dioxide from the trenches, and gases formed from decomposing bodies and from exploding ammunition had all fused to form a highly toxic vapor that the victims had inhaled. Among other causes of the epidemic advanced by physicians were the effects of cosmic rays, atmospheric pressure, air stagnation, ozone, coal dust, fleas, the distemper of cats and dogs, and dirty dishwater.
Late in September, in the unlikely setting of the National Swine Breeders’ show in Cedar Rapids, Iowa, about 45o miles from Fort Riley, Kansas, a more likely speculation was being formulated. Hog breeders at the show were alarmed to find their prize animals felled by sickness—prostrate, sweating, and groaning in the display pens. Dr. J. S. Koen, an inspector from the Division of Hog Cholera Control of the Bureau of Animal Husbandry, thought he detected a similarity between the hogs’ ailment and the influenza now striking people in Cedar Rapids. He identified the sickness in his report to Washington as the “hog flu.” A decade later researchers would conclude that there was indeed a connection between the two diseases.
When influenza assailed human beings, it struck like a thunderclap. One minute a person would be perfectly well and then, suddenly, would feel totally helpless, overcome with lassitude. The victim wanted to stop whatever he or she was doing and lie down immediately. Early symptoms were headache, chills, fever, and great pain in the legs and back. The inside of the mouth and throat turned flaming red. The influenza patient often coughed agonizingly, and breath came in painful sobs. Frequent complications were paralysis of eye muscles, swollen ankles, and blood-streaked urine. As the infection deepened, symptoms in some cases resembled those of encephalitis, with the victim slipping into a coma.
But it was the devastation dealt the lungs that killed. An albuminous discharge filled the air sacs in the lungs, preventing the capillaries from taking vital oxygen to the body. Thus the actual cause of death was asphyxiation. The results of autopsies were appalling: the lungs showed evidence of hemorrhaging and abscessing, swelling, accumulation of pus, sometimes total collapse. The influenza often would open the gateway to fatal complicating diseases, particularly pneumonia. Yet for all its deadly force, patients who did not succumb to the illness were usually up and about in a week or so.
In earlier outbreaks influenza, like other respiratory diseases, had tended to take the lives of the weak and the old. Pneumonia, for example, has commonly been called the old man’s friend because it carries off the elderly quickly, almost painlessly. But the plague of 1918 had a cruel twist. Like the war it paralleled, the Spanish influenza killed off mostly the young and the strong. The disease felled young fathers and mothers in droves. In one week in October, 19 out of 42 women in a San Francisco maternity ward died of influenza. Yet during the outbreak the death rate among those aged forty-five to seventy-four tended to remain where it had been before the epidemic.
In the years just preceding the epidemic, the influenza and related pneumonia death rate among white men between twenty-five and thirty-four had been about 80 deaths per 100,000. In 1918 that rate rose to nearly 2,000, or 2,400 per cent. For white women death struck hardest at those between twenty and twenty-four, raising the influenza death rate by 2,500 per cent over the previous period. Influenza deaths also rose among blacks, though the rate was only about one fourth of that for whites.
Army doctors noted a curious distinction early in the outbreak at Fort Dix. Men from tough big-city neighborhoods seemed almost immune, while brawny farm boys from the blue-skied wide-open spaces fell like tall trees. The doctors concluded that men with large, well-developed chests breathed in more germs than their slighter comrades. They reasoned further that their strong constitutions also unleashed a massive counter-attack that killed off the invading organisms too swiftly and thus somehow proved overwhelming to the human system. The stronger the body, the swifter seemed its collapse.
Not knowing the true identity of the enemy, medical scientists were hard pressed to devise a vaccine in defense. Some people were inoculated with a mixture of organisms from influenza patients. Some were vaccinated with diphtheria antitoxin and antitetanus and antimeningitis sera. Doctors at a Boston naval hospital injected patients with a mixed vaccine that included Pfeiffer’s bacillus, the germ believed to have caused an influenza epidemic in 1890. This experiment produced some encouraging results. A few other successes with vaccines were also reported. On the whole, however, the results of vaccination were inconclusive, since they were used on an infinitesimal number of the total population. A committee of pathologists appointed by the Massachusetts State Board of Health, either unaware of or unable to duplicate the results of the Boston naval hospital experiment, reported that “the evidence at hand convinces the board that the vaccines we have considered have no specific value in the treatment of influenza.”
Even attempts to verify the way the disease spread were frustrated. On a parade ground at the naval prison in Boston Harbor’s Deer Island, Dr. Joseph Goldberger, a public-health physician, stood before a thousand deserters, insubordinates, brawlers, and other delinquent sailors. The doctor needed volunteers for an influenza experiment. The epidemic had stricken nearly a quarter of the total personnel and was on its way to taking the lives of 5,000 men.
Goldberger was blunt. Volunteers would first have to inhale a pure culture of Pfeiffer influenza bacillus into their nostrils. If this did not infect them, they would be injected with matter from the lungs of dead influenza victims. Volunteers would next have secretions from influenza patients sprayed into their nostrils and eyes and swabbed inside their throats. Finally each volunteer would be assigned to a seriously ill influenza patient who would cough directly into his face.
In exchange for running this gauntlet with death all surviving volunteers would be pardoned and restored to active duty. Three hundred men volunteered; Goldberger took 62 of them for his experiment.
A week after the sailor guinea pigs had sniffed, been injected, had their throats swabbed, and 10 of them had inhaled the coughs of influenza victims, not one man had contracted the disease. Their escape may be explained by the fact that the epidemic had already passed through Deer Island before the experiment had started, and these particular volunteers may have possessed natural immunity. Ironically, the doctor in charge of the quarantine station where the experiment was conducted did contract influenza and died during the study. But as for the experiment itself, it failed to verify the one point that the doctors previously had thought they were sure of—how the disease was transmitted.
The millions of Americans sick and the tens of thousands dead affected both the civilian and the military war effort, which of course went on despite rumors of an imminent armistice. Massive absenteeism slowed production at shipyards and arsenals. As early as September 27 the New York Times had reported: “Influenza Stops Flow to the Camps of Drafted Men.” Orders for 142,000 men scheduled to report for induction the following week were canceled, since draft officials were understandably reluctant to pump manpower into the hothouses of infection that the army camps had become. In the first week of October 16,000 military men were stricken, and troopships headed for France became floating pestholes. In the American Expeditionary Force roughly one out of every three soldiers with influenza died, far worse odds than a man faced in battle. Among some army units influenza mortality reached 80 per cent.
The raging course of this pestilence posed a cruel dilemma for the nation’s Commander in Chief. On the evening of October 8 Woodrow Wilson met with the army chief of staff, gruff, able General Peyton March.
“General March,” Wilson said, “I have had representations made to me, by men whose ability and patriotism are unquestioned, that I should stop the shipment of men to France until the epidemic of influenza is under control.” March was well aware that at embarkation ports where 300,000 troops were being sent through monthly, the death rate among the sick had reached ao per cent. To pack soldiers into troopships, both men knew, was to pass a death sentence on thousands of them.
But the epidemic was siphoning off manpower at both ends of the military pipeline; all told, 70,000 A.E.F. doughboys were now sick. Their commander, General “Black Jack” Pershing, pleaded desperately for replacements. American troops were winning the battle of the Meuse-Argonne against the Germans in their sector, despite over 150,000 down with influenza. Were they to lose it now to a germ? Wilson had to weigh the effect of a troop cutoff on Pershing’s staying power.
One additional and critical factor had to figure in Wilson’s decision. A few days before the conference with General March, Max, prince of Baden, imperial chancellor of Germany, had appealed to Wilson for an armistice. The enemy finally seemed to be cracking. Wilson had to estimate the effect on the enemy’s faltering will to fight should the Germans suddenly learn that the pressure was off, that the flow of American replacements had ceased.
Of course, the flu struck both sides with a harsh neutrality. General Ludendorff dreaded the dreary morning recitals by his chiefs of staff of the latest count of troops felled by the disease. Many German companies were down to fifty rifles, and the plague was obviously sapping the strength of Germany’s army and its people. Civilian and military deaths from the epidemic were moving toward a quarter of a million. But all this was reason for the Allies not to relent, but to tighten the screws on their staggering foe.
Wilson is reported to have said: “They tell me you decline to stop the shipments.” March replied: “Every such soldier who has died [from influenza] has just as surely played his part as his comrade who has died in France. The shipment of troops should not be stopped for any cause.”
The President thought gravely about it and finally agreed with March to keep the troops going. Then, just as March was about to leave, Wilson’s eyes brightened, and the general was astonished to hear the President of the United States recite:
Children were spouting the same rhyme all over America that grim fall.
In the face of unremitting devastation, the ineffectual correctives went on: the arrests for spitting and coughing, the use of face masks, the prohibitions on public gatherings, and the desperate cures—inhalation of chloroform fumes, removal of tonsils, even the extraction of teeth. In Louisiana the presumed medicinal properties of Scotch whisky drove the price up to twenty dollars a quart. In New York City the telephone company pleaded with people to limit calls to urgent matters, since the epidemic had not only loaded the circuits with many genuine emergencies but had at the same time decimated the ranks of telephone operators.
There was no reluctance to tie influenza to the Great War. Evangelist Billy Sunday exhorted his flock: “We can meet here tonight and pray down an epidemic just as well as we can pray down a German victory. The whole thing is a part of their propaganda; it started over there in Spain, where they scattered germs around … there’s nothing short of hell they haven’t stooped to since the war began.”
Lieutenant Colonel Philip Doane, who was in charge of the health and sanitation section of the Emergency Fleet Corporation, a merchant-ship auxiliary set up during the war, speculated: “We know that men have been ashore from German submarine boats. It would be quite easy for one of these German agents to turn loose Spanish influenza germs in a theatre or some place where large numbers of persons are assembled.”
A rumor spread swiftly that a group of medical-corps officers and nurses at Gamp Hancock, New Jersey, had been lined up before a firing squad and executed as German spies. They had been convicted, word had it, of spreading influenza by hypodermic injection of the troops. The story proved so stubborn that the Army’s acting surgeon general, Brigadier General Charles Richard, felt compelled to issue a denial: “The reports are ridiculous and without the slightest foundation in fact.” Another story had German agents holed up in Chevy Chase, Maryland, brewing batches of influenza germs.
In the latter part of October the Spanish-influenza epidemic whipped to a furious crescendo. The week of October 23 witnessed the death of 21,000 Americans, the highest mortality toll ever recorded in America at any time for any cause. In the state of New York alone 45,000 people had perished since the outbreak began. On October 23, 851 people died in New York City of influenza, the highest daily figure ever registered there. For days the number of deaths held at the 800 level. An even more staggering loss took place in Philadelphia. In the previous week 5,270 had died, a rate 700 times above normal. Of these deaths all but a few hundred were attributed to influenza and related pneumonia.
Then, with the beginning of November, the plague began to pass as inexplicably as it had arrived. The number of new influenza cases plummeted. In Philadelphia the city’s health chief declared that the epidemic “has ceased to exist officially,” so the bars were reopened—and almost immediately 53 persons were arrested for disorderly conduct and drunkenness. In Washington, President Wilson was in the audience as B. F. Keith’s theatre resumed with a new show, An American Ace . “Flo-Flo” and her “Perfect 36” chorus ended their emergency work in a Washington hospital and took up their accustomed vocation in Chicago. In homes across America windows were flung open, floors scrubbed, rugs aired, and face masks tossed out, and various flu remedies were shoved to the back of the medicine cabinet.
Thus, with an irony that surpasses man’s understanding, the plague faded almost simultaneously with the ending of the Great War. Except for a few belated stabs into remote corners of the earth, the worst of the Spanish-influenza epidemic was over. It seemed almost as though nature had determined to show man who still reigned supreme in the giving and taking of life: the war had killed over 21 million people in four years of dogged conflict; the influenza epidemic took approximately the same toll in about four months.
In all history there had been no sterner, swifter visitation of death. The plague of Justinian, beginning in the year A.D. 542, had supposedly claimed 100 million lives—but it endured for fifty years. The bubonic plague, the Black Death of the fourteenth century, was said to have taken a quarter of Europe’s population, 25 million lives, and another 37 million more in the East; it lasted for three years. Spanish influenza held sway for about a hundred and twenty days. Mathematically, had the epidemic continued its rate of acceleration, humanity would have been eradicated in a matter of months.
In the United States the final reckoning was 548,452 lives lost. Nearly ten times as many Americans died in those few weeks of pestilence as had been killed in eighteen months of war. Modern vital statistics provide an illuminating comparison. In 1978, for every 100,000 persons in the population, 167 died of cancer and 494 of heart-related disease. In 1918 the comparable figure for those dying of influenza and related pneumonia was 588, a mortality rate for this country never approached before or since.
Insurance-company actuarial tables were left a shambles by the epidemic. The Metropolitan Life Insurance Company paid out over $ 18 million to beneficiaries of 85,000 policies in a few months. No company in the history of insurance had ever experienced a remotely comparable drain of assets.
The precise cause of this scourge was never positively identified at the time. Not until fifteen years later did a group of British researchers succeed in infecting laboratory animals with influenza germs. This advance made it possible, for the first time, to study the disease and to develop preventive approaches. At the same time it was discovered that the germ entered the body solely through the nasal passages. And in 1943, using the newly-developed electron microscope, a team of U.S. researchers finally saw an influenza virus, a miniature cottonlike ball so small that 30 million would fit on the head of a pin. Still, this virus was only a particular influenza strain and not necessarily the killer of 1918.
In 1951 Dr. Albert P. McKee of the State University of Iowa led a research team to Alaska on an ingenious course of inquiry that held out great promise of finally unmasking the killer virus. The scientists exhumed the bodies of several Eskimos who had died during the 1918 epidemic. Their bodies, buried inside the permafrost line, were well preserved. The researchers sliced lung sections from these longgone victims and sent the tissue back to Iowa City, hoping that laboratory animals could be infected if the virus were still partially animated. If this happened, the virus could then be isolated and identified. But whatever germ had killed these Eskimos in 1918 had disappeared. The mystery persisted.
Researchers did learn later that influenza is caused by viruses possessing a remarkable capacity for change. The Australian microbiologist Sir Frank MacFarlane Burnet concluded: “Of all the virus diseases, influenza is probably that in which mutational changes in the virus are of greatest human importance … the influenza virus’ chief means of survival is its capacity for constant mutation. …” The influenza-virus particle, a speck of nucleic acid in a “protein coat,” is capable of continually rearranging its structure to form new strains.
This mutability may explain the savagery of the 1918 outbreak, along with another kind of adaptability possessed by the virus. Ten years after the epidemic a virologist at the Rockefeller Institute, Dr. Richard Shope, concluded that a mild hog virus acting in combination with another equally mild human influenza virus may have produced a virulent new strain. Two sicknesses, by themselves merely discomforting, may have been fused through a kind of viral synergism to produce the deadliest plague in human history. Most influenza experts eventually agreed that the sickness of the swine and that of human beings were somehow linked in the great epidemic of 1918.
Could it happen again? Virologists say Yes. If animal and human viruses joined disastrously before, they could do so again. Given the rapid spread of such a disease, given the time required to identify the new invader, to devise an effective vaccine, and to immunize the population, it is conceivable that another epidemic could strike with the same force. However, vaccine research is actively under way; and in any event medical scientists believe that modern drugs and better medical care overall would prevent death on the horrifying scale of the 1918 plague.
As for that viral holocaust, the most able medical men of the time admitted that all their efforts had probably not limited the epidemic by an hour. One doctor called it the medical catastrophe of all time. Why it began, why it ended, where it went, no one knows to this day. Like a malign comet, the Spanish-influenza epidemic of 1918 sped over the earth, took 21 million lives, and vanished.