In terms of the massive number of deaths within a very short period of time, the 1918 influenza pandemic was the single worst illness ever to hit humanity. The Spanish Flu, or "La Grippe" as it was sometimes known, killed more people than the Great War (WW I) which lasted from 1914 to 1918 and killed 9 million men over those years.
By comparison, the bubonic plague, at its peak, claimed only 2 million victims a year. The influenza pandemic claimed 20-70 million people within a year.
America was one of the least devastated countries on the planet but, even so, the influenza pandemic claimed more victims in the U.S.A. than all of their armed conflicts of the 20th century with approximately 850,000 deaths from 20 million cases. Of the U.S. soldiers who died in Europe roughly half fell to the disease. In equivalent numbers, if it happened today, 1.5 million Americans would die of the disease.
The virus infected one fifth of the world's population and was most deadly for people aged 20-40, a change from normal influenza which is largely a killer of young children and the elderly. The mortality rate from the 1918 influenza strain was 2.5% compared with 0.1% from typical influenza and drove the American life expectancy rate in 1918 down by twelve years. The actual cause of death was often not the influenza itself but the pneumonia accompanying the infection.
People without symptoms could be struck suddenly and be rendered too feeble to walk within hours. Many would die the next day.
Symptoms began with headaches and feverish chills. The sinuses and lungs of the victim would fill with greenish-yellow mucous and the victim would begin to suffocate in the fluid. The skin would turn blue or purple and extremitites could turn black from oxygen deprivation. Soon the patient would be coughing up blood-soaked phlegm before quietly drowning in bed.
As with the bubonic plagues of earlier centuries, the disease was spread by human carriers along trade routes and shipping lines. The mass movements of the troops to and from the war aided the rapid diffusion of the disease and the cramped communal conditions the men were exposed to, even before they reached the front line, made the job of the virus all the easier.
The name "Spanish Flu," by which the pandemic has been commonly known, came from early mortalities in Spain where the virus allegedly killed eight million people in May 1918. However, the disease first struck in Camp Funston, a U.S. Military camp in Kansas.
On the morning of March 11, 1918, company cook Albert Mitchell reported to the infirmary with flu-like symptoms and was recommended bed rest. By noon 107 soldiers were sick. Within two days the number had risen to 522, many with severe pneumonia. The disease was soon reported in other military bases around the country. Sailors docked off the East Coast were sick in the thousands and isolated places like Alcatraz prison were also infected. The disease had to be airborne. Within just seven days, every U.S. state was affected.
The virus then spread with the soldiers across the Atlantic infecting Allied troops and civilians. By mid-April, China and Japan had the disease as well.
The only positive point of the disease was that it usually peaked within a few weeks of appearing in a city and disappeared as quickly as it arrived. Unfortunately for the U.S., the war brought the disease they had begun back to them, arriving in the port of Boston in September 1918. Men across the country were mobilizing for the war and when they came together the virus was able to infect thousands more. 200,000 people, mainly soldiers, died in October alone.
Many countries and local governments enforced restrictions on tracel and public gatherings. In many places, public gathering spaces like dance halls and theatres were closed for over a year.
When the world celebrated the end of four years of war in November 1918, the Armistice Day celebrations became a public health disaster. Thousands of people came to watch parades and join in huge parties where they became infected by sick soldiers returning from the front line where the disease had killed more people than their weapons.
Hospitals around the world had their resources stretched to the breaking point. Many of the doctors were at the front lines treating soldiers and some of the doctors jobs at home were filled by third and fourth year medical students who hadn't joined the armed forces. Many nurses also quickly fell victim to the disease and soon there was a massive shortage of trained health care providers to care for the sick, not to mention a lack of able-bodied grave diggers to bury the dead.
The only inhabited place of any size with no documented outbreak of the disease in 1918 was the island of Marajo at the mouth of the Amazon river in Brazil. The rest of the world was not so fortunate and several towns around the world simply ceased to be when their entire population was wiped out by the virus.
Eighteen months after the disease first struck, it disappeared. Medical science in 1918 held no understanding of what was happening and most efforts at the time were spent in an unsuccessful quest to find a germ-borne cause of the disease. Ironically, many people believed it was somehow caused by germs from the large number of decaying bodies of soldiers killed in the war. Some believed it was the result of germ warfare. No one knew exactly what had really happened until very recently.
In 1997, the lungs of an eighteen-year-old soldier killed by the disease in 1918, and exhumed from a grave in the Alaska permafrost, were used to obtain a partial genetic code of the virus.
Only seven percent of the virus' code was actually recovered but it was enough to show that the disease started as a virus affecting birds. Instead of remaining stable in the bird population, the virus mutated and leaped to pigs. The pig immune system forced the virus to mutate again in order to survive, and from there it infected humans.
This kind of virus is the worst as there is no natural immunity to it in the human body.