On September 28, 1918, a Liberty Loan parade in Philadelphia prompts a huge outbreak of Spanish flu in the city. By the time the pandemic ended, an estimated 20 million to 50 million people were dead worldwide.

Influenza is a highly contagious virus that attacks the respiratory system and can mutate very quickly to avoid being killed by the human immune system. Generally, only the very old and the very young are susceptible to death from the flu. Though a pandemic of the virus in 1889 had killed thousands all over the world, it was not until 1918 that the world discovered how deadly the flu could be.

The most likely origin of the 1918 flu pandemic was a bird or farm animal in the American Midwest. The virus may have traveled among birds, pigs, sheep, moose, bison and elk, eventually mutating into a version that took hold in the human population. The best evidence suggests that the flu spread slowly through the United States in the first half of the year, then spread to Europe via some of the 200,000 American troops who traveled there to fight in World War I. By June, the flu seemed to have mostly disappeared from North America, after taking a considerable toll.

Over the summer of 1918, the flu spread quickly all over Europe. One of its first stops was Spain, where it eventually became known the world over as the Spanish flu. The Spanish flu was highly unusual because it seemed to affect strong people in the prime of their lives rather than babies and the elderly. By the end of the summer, about 10,000 people were dead. In most cases, hemorrhages in the nose and lungs killed victims within three days.

As fall began, the flu epidemic spiraled out of control. Ports throughout the world—usually the first locations in a country to be infected—reported serious problems. In Sierra Leone, 500 of 600 dock workers were too sick to work. Africa, India and the Far East reported epidemics. The spread of the virus among so many people also seems to have made it even more deadly and contagious as it mutated. When the second wave of flu hit London and Boston in September, the results were far worse than those from the previous flu strain.

Twelve thousand soldiers in Massachusetts came down with the flu in mid-September. Each division of the armed services was reporting hundreds of deaths each week due to flu. Philadelphia was the hardest-hit city in the United States. After the Liberty Loan parade (celebrations to promote government bonds that helped pay for the Allied cause in Europe) on September 28, thousands of people became infected. The city morgue, built to hold 36 bodies, was now faced with the arrival of hundreds within a few days. The entire city was quarantined and nearly 12,000 city residents died. Overall, in the United States, five out of every thousand people fell victim to the flu.

In the rest of the world, the death toll was much worse. In Latin America, 10 out of every thousand people died. In Africa, it was 15 per thousand and in Asia it was as high as 35 per thousand. It is estimated that up to 20 million people perished in India alone. Ten percent of the entire population of Tahiti died within three weeks. In Western Samoa, 20 percent of the population died. More people died from the flu than from all of the battles of World War I combined.