The deadly viruses that vanished without trace

Instead, Sars disappeared as abruptly as it arrived. By January 2004, there were just a handful of cases – and by the end of month, the last suspected natural infection was announced. Oddly, while “patient zero” describes the first known person to be infected with a virus, there is no equivalent label for the last ever person to catch it in the wild. But this would arguably apply to a 40-year-old man with the family name of “Liu” from the southern city of Guangzhou. (There was another outbreak a couple of months later, when it is thought to have escaped from a Beijing research lab – twice).

So what happened?

In a nutshell, we got lucky. According to Sarah Cobey, an epidemiologist at the University of Chicago, Sars was driven to extinction by a combination of sophisticated contact-tracing and the quirks of the virus itself.

When patients with Sars got sick, they got very sick. The virus had a staggeringly high fatality rate –almost one in five patients died – but this meant that it was relatively easy to identify those who were infected, and quarantine them. There was no extra spread from people without symptoms, and as a bonus, Sars took a relatively long time to incubate before it became contagious, which gave contact-tracers extra time to find anyone who might be infected before they could pass it on.