Breathe easy: how respiratory viruses evolve to become milder

The Queen has suffered ‘mild, cold-like symptoms’ from her Covid-19 infection, according to Buckingham Palace. The wording reminds us that, except in the very vulnerable, the common cold is always and everywhere a mild disease. There are 200 kinds of virus that cause colds and they hardly ever debilitate healthy people, let alone kill them. Yet we were recently told by the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) that ‘it is a common misconception that “viruses mutate to cause less severe disease”’. If that is the case, how did all common colds become mild — and why would Covid not do the same?

As somebody with a background in evolutionary biology, I knew that Nervtag’s claim (which bizarrely cited myxomatosis, a flea-borne disease of rabbits, to support its argument) was misleading. Surely they were aware that, mostly due to the work of Professor Paul Ewald, the dominant belief in evolutionary theory about disease virulence is that it depends on the mode of transmission? Though sometimes lethal at first, respiratory diseases do evolve to become milder, while sexually transmitted, waterborne or insect-borne diseases (such as myxomatosis) don’t.

And ‘evolve’ is the right word, not ‘mutate’. The way medical scientists talk about evolution is sometimes alarmingly naive, as if random mutation is what drives it. No, no, a thousand times no: it’s selection. For example, I took a train this week, putting me at risk of catching Covid from a fellow passenger. But if two other people had been planning to travel on the same train, one with mild omicron and the other with severe delta, the latter would have been more likely to change their mind and stay home because of feeling unwell. That’s selection. The fiercest enemy of a virus is another virus. Omicron ousted delta at least partly because people with mild symptoms were more likely to go to work or parties (or not notice they were ill) than people with severe ones.