Evolutionary Just-So Stories About Viruses and Virulence

An evolutionary just-so story is a plausible sounding but unfalsifiable explanation for a trait or behavior of an organism. What follows will be one such.

The things that make you feel sick are a virus’s way of spreading to new bodies. A virus that multiplies inside you without causing any kind of effluvia will just sort of hang around until your immune system sees that something is up and snuffs it out. A virus that makes you sneeze, on the other hand, has a nice mechanism for jumping to the next body. Likewise for vomiting, diarrhea, sores, pustules, fever sweats, etc. Unfortunately, these things also make you really sick and can kill you if they’re bad enough. A virus that makes you over-produce mucus will make you sneeze, cough, wipe your nose and touch other surfaces, and so on. It might also give you a fatal pneumonia. A fever might make you sweat (do any viruses actually spread via sweat?), but too high a fever will kill you.

Viruses that kill their hosts quickly don’t do a very good job of multiplying. Even viruses that cause obvious symptoms of illness are bad at spreading, because they cause people to stay home and cause other people to keep their distance. This is particularly true if there is a deliberate effort to quarantine anyone with a specific set of symptoms. The virus that is best at spreading will find a sweet spot. “Don’t make my host so sick he’s bed-ridden or dead. Make him just sick enough that he smears his spittle on a few door-handles.”

Viruses mutate. Mutations will generally create variation in the virus' traits, including the severity of symptoms in its host. The mutations that make a virus less deadly will be better at spreading. Longer onset and lower severity of symptoms make for a more effectively spreading virus. If viral mutation is causing variation in the severity of symptoms, this will tend to make the virus less severe over time. (I don’t know if this generally takes place on the order of weeks or years or decades, but something like this happens with real viruses.) If you are a viral strain that gives people a few sniffles and the occasional sneeze, your hosts will be out and about spreading your offspring. Your “cousins” that leave people bed-ridden or kill their hosts will likely die off.

I recall a documentary about the introduction of the cane toad to Australia. It was introduced to control the population of cane beetles, another nasty pest. But it rapidly became an invasive species. It spread across the continent and is still spreading to this day. The interesting part of this story is that cane toads at the edge of its expanse tended to have longer legs than cane toads closer to the point of introduction (some sugar plantations north of Queensland). This makes sense. If cane toads are hopping around at random in search of habitat, the ones with the longest legs will have traveled the furthest. They will find other long-legged toads to mate with at the frontier and have long-legged offspring. The analogy here is that viruses with mild symptoms have “longer legs” than their more virulent strains.

Obviously I am hoping that something like this is happening with the coronavirus. I don’t know if that’s likely, or if it is how long it will take. It seems like the median case is mild or asymptomatic, and the reason it's so scary is that there is a small but non-trivial chance of a very bad outcome. The above story makes me skeptical that asymptomatic spread is a big deal. If you're not coughing or sneezing, the virus doesn't have as many avenues of escaping to the next body. (Unless it's like the infamously contagious measles, where an infected person can contaminate a room for hours just by breathing.)  It's still possible, just less likely.