Human history has seen many viruses before COVID-19, but what are the deadliest viruses the world has faced? 

Biochemist Professor Luke O’Neill said all of human history “has been affected by viruses”. 

“There’s a view out there that human history was governed by them, because they will often wipe out loads of people in a population,” he said on Show Me the Science.  

Prof O’Neill said the typical measure of a virus’ deadliness is the ‘case fatality rate’ - the percent of people who are infected with a virus that die. 

“That’s the only metric,” he said. “You can have a virus that may not have a very low case fatality rate, let's say 5%, but if it’s highly transmissible and millions and millions get infected, the total number of deaths will be high.” 

Covid-19 particles in an illustration. Picture by: Science Photo Library / Alamy Stock Photo

Across history, it’s also important to consider the death rate in relation to the Earth’s population at the time – if 10,000 out of 20,000 people die of a virus, that can be considered a deadlier virus than one that kills one million out of five million people. 

“It's a complicated thing to pin down,” Prof O’Neill said.

Ebola and Marburg

All things considered, many agree Ebola and Marburg are among the deadliest viruses in the world. 

“They range from 50, 60, 70 up to 90% case fatality rate,” Prof O’Neill said. “If you’re infected with Marburg, there’s a 90% chance you’re going to die. 

While these viruses have a high case fatality rate, however, the lower risk of transmission means overall death rates are not dangerously high. 

“The good news is you can tell people who have these viruses and it's only spread from people who are really, really sick. 

“You can contain it.” 

Rabies

Another commonly agreed deadly virus is rabies. 

“The overall case fatality rate for rabies can be as high as 50%,” Prof O’Neill said. “There’s a 50/50 chance of survival. 

“If you got rabies 100 years ago, before any vaccines or treatments, there was a very high 50% case fatality rate.” 

Influenza

The 1918 influenza pandemic killed 50 million people worldwide – more than World War II – and has its own unique deadliness. 

“The strange thing about the 1918 pandemic was it was younger people dying,” Prof O’Neill said. “Normally they have a very powerful immune system. 

Experts theorise young people died more because their immune was in “overdrive”. 

“If you had very strong troops in your immune system, and they really began to get activated, you’re dying of an overactive immune system,” Prof O’Neill said. 

Vaccines

Most viruses in today’s world now have vaccines and treatments to lower their fatality rate, such as HIV, smallpox and most recently COVID-19. 

“The vista of hundreds and hundreds and millions of people dying that happened in history is lessened because of the great science that has happened,” Prof O’Neill said. 

Click here to listen back to Show Me the Science with Luke O'Neill.