Question: What is the most deadly virus?

Stephanie Moon answered on 1 May 2016:

Hi Jesse,
Thanks for the great question! So, I would say that to be the deadliest virus we should rate viruses based on three things: how many people have died from the virus over time, how many people that get infected with the virus die (called a mortality rate), and how easy it to get the virus (how infectious is it)? The virus that causes smallpox might be the worst in history, but it’s been eradicated (nobody has actually gotten smallpox since the 1970’s thanks to a global vaccination campaign). It’s estimated that 300 million people died in the 1900’s, three times more deaths than those caused by war… and we think smallpox was infecting people for thousands of years before then, with about a 30% death rate (1 of 3 infected people would die). Compare that to Dengue virus, which infects a lot of people every year (about 200-400 million people per year) but only about 20,000 people die each year from it. Then there’s the Ebola and Marburg viruses– they can kill 50-90% of people that are infected. Rabies is worse- if you get rabies (usually from being bitten by an infected animal) and if you don’t get a shot right away (luckily we have a treatment for it!) it’s basically not survivable. These three viruses have high mortality rates but they aren’t that easy to get- they aren’t airborne for example, so they’re less contagious. Influenza virus on the other hand is quite contagious and can be deadly (in 1918 flu killed 50 million people, and half a million people still die each year from flu), but we have treatments and vaccines for it. So, if I had to choose one virus as the world’s most deadly virus, I think I would say smallpox.