If these numbers shock, it’s because today epidemics are rarely discussed in history classes, while in the not so distant past, they were simply a terrible fact of life. There are few memorials to the victims of disease. The historian Alfred Crosby was the author of America’s Forgotten Pandemic, one of the great books on the 1918 flu. But Crosby was only prompted to begin researching the pandemic when he stumbled on the forgotten fact that American life expectancy had suddenly dropped from 51 years in 1917 to 39 years in 1918, before rebounding the following year. That plummet in 1918 was because of a virus just 120 nanometers wide.

Viral advantage

Pathogens make such effective mass killers because they are self-replicating. This sets them apart from other major threats to humanity. Each bullet that kills in a war must be fired and must find its target. Most natural disasters are constrained by area: an earthquake that strikes in China can’t directly hurt you in the UK.

But when a virus – like the novel coronavirus – infects a host, that host becomes a cellular factory to manufacture more viruses. Bacteria, meanwhile, are capable of replicating on their own in the right environment.

The symptoms created by an infectious pathogen – such as sneezing, coughing or bleeding – put it in a position to spread to the next host, and the next, a contagiousness captured in the replication number, or “R0” of a pathogen, or how many susceptible people one sick person can infect. (Imperial College London has estimated the novel coronavirus’s R0 at 1.5 to 3.5.) And because human beings move around – interacting with other human beings as they do so in every manner from a handshake to sexual intercourse – they move the microbes with them. (Read more about what makes viruses so difficult to outsmart).

No wonder militaries have long tried to harness disease as a tool of war. No wonder that, until recently, far more soldiers died of disease than died in combat. A pathogen is a perfectly economical weapon, turning its victims into its delivery system.