The media attention given to influenza has clothed this disease in the fabric of modernity. But influenza is, in fact, a very old disease that dates back thousands of years and varies in lethality from year to year and outbreak to outbreak.

In ancient Greece, Hippocrates described the symptoms of flu roughly 2400 years ago. For historical events such as these, we primarily have descriptions of symptoms rather than accurate diagnoses or demographic data. This makes it hard to accurately pinpoint ancient flu pandemics. One of the earliest records of an outbreak that can be linked to influenza was in 1580. It began in Russia and ultimately spread to Europe by way of Africa. One of the more virulent strains, this influenza killed 8,000 people in the city of Rome alone. Many villages in Spain were totally decimated.

Pandemics seem to hit about every century. There were periodic outbreaks in the 17th and 18th centuries. The pandemic of 1830 was especially widespread. It was also particularly infectious, and one out of four people exposed, or 25%, contracted the disease after exposure.

But the most infamous of all influenza outbreaks was the pandemic of 1918. Estimates for the number killed in this outbreak were between 20 and 100 million people. The wide range in the estimate is due to the global nature of this outbreak. This outbreak reached far flung Pacific Islands and the Arctic.

Not only did this strain of influenza have an extremely high infection rate—over 50% of people exposed contracted the disease—but the symptoms were especially severe. Hemorrhaging from the mouth, nose and intestines were common, as were hemorrhages under the skin. A majority of deaths were from secondary infections like pneumonia, but the virus killed directly with massive bleeding and swelling.

Another unusual feature was that the highest mortality rate was among young, otherwise healthy, adults. This was linked to a cytokine storm, which is a condition where the normal antibody/cytokine cascade is not stopped by the body’s own processes. It is as if the body over responds to the contagious agent, making so many antibodies that they begin to harm the body. Say this happens in the lungs. The lungs will fill with fluid and suffocate the patient before the virus can kill the patient.

Having established that the 1918 pandemic was bad, just how bad was it? It is estimated that between 2.5% or more of the entire world’s population was killed. To put in another way, in the first 25 years of the AIDS epidemic, 25 million people died. In 1918, over 25 million died in 25 weeks.