Mutation unknown: Covid’s mysterious evolution

Common wisdom states that a new pathogen, once introduced into a vulnerable human population with no immune defences, will evolve over time to grow more benign and live in amity with its host. After all, the argument goes, a pathogen will depend on its host for its own survival, so causing mass disease and death will make onward transmission more difficult and ultimately only ensure its own demise.

Unfortunately, despite the reassuring logic, the reality is often more nuanced. Infection by water or food-borne pathogens can commonly result in severe or even life-threatening diarrhoea. However, far from disadvantaging the pathogen, this symptom can actively facilitate onward transmission via the faecal-oral route. Malaria is a vectored disease (those that are transmitted by living organisms) that is spread via mosquitos. In this case, the extreme sickness suffered by the host won’t impede onward transmission if there are plenty of healthy mosquitos around to help the parasite jump ship from one host to the next.