Throughout history, nothing has killed more human beings than infectious disease. Covid-19 shows how vulnerable we remain – and how we can avoid similar pandemics in the future.
Covid-19 marks the return of a very old – and familiar – enemy. Throughout history, nothing has killed more human beings than the viruses, bacteria and parasites that cause disease. Not natural disasters like earthquakes or volcanoes. Not war – not even close.
Take the mosquito-borne disease malaria. It has stalked humanity for thousands of years, and while death tolls have dropped significantly over the past 20 years, it still snuffs out nearly half a million people every year.
Over the millennia, epidemics, in particular, have been mass killers on a scale we can’t begin to imagine today – even in the time of the coronavirus.
The plague of Justinian struck in the 6th Century and killed as many as 50 million people, perhaps half the global population at the time. The Black Death of the 14th Century – likely caused by the same pathogen – may have killed up to 200 million people. Smallpox may have killed as many as 300 million people in the 20th Century alone, even though an effective vaccine – the world’s first – had been available since 1796.
Some 50 to 100 million people died in the 1918 influenza pandemic – numbers that surpass the death toll of World War One, which was being fought at the same time. The 1918 flu virus infected one in every three people on the planet. (Read more about how the 1918 flu changed the world). HIV, a pandemic that is still with us and still lacks a vaccine, has killed an estimated 32 million people and infected 75 million, with more added every day.
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.)
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.
A better era
In the developed world, and increasingly in the developing world, we are now far more likely to die from non-communicable diseases like cancer, heart disease or Alzheimer’s than from a contagion. The decline of infectious disease is the best evidence that life on this planet truly is getting better.
Not over yet
That’s the good news. The bad news, as Covid-19 reminds us, is that infectious diseases haven’t vanished. In fact, there are more new ones now than ever: the number of new infectious diseases like Sars, HIV and Covid-19 has increased by nearly fourfold over the past century. Since 1980 alone, the number of outbreaks per year has more than tripled.
There are several reasons for this uptick. For one, over the past 50 years, we’ve more than doubled the number of people on the planet. This means more human beings to get infected and in turn to infect others, especially in densely populated cities. We also have more livestock now than we did over the last 10,000 years of domestication up to 1960 combined, and viruses can leap from those animals to us.
(This is an abridged version of a long article published by the same name on the BBC World’s Website. Click to read the original story.)