The time scale of viral evolution is hours. The infectious vectors called virions can each produce thousands of copies of themselves from a single infected cell. Within hours, natural selection will determine which virions will successfully infect another cell/organ/individual; and which will die.

In ecology, the opposing reproductive strategies of wide dispersal and rapid colonization vs. taking a stand and extracting maximal resources are mathematically modeled as r and k selection, respectively. R selection favors being a weed. K selection favors being a giant sequoia.

The hypothesis of seasonal Darwinism attempts to explain the mysterious seasonality of some viruses as near real time r selection, favoring dispersal and transmissivity, during the summer when human populations are better fed, more dispersed, more outside, and more vitamin D enriched; and k selection, favoring virulence, in the winter when these factors are reversed.

SARS 2 is a single stranded positive RNA virus. It must first create a negative copy for replication, making replication and translation at the same time more difficult. The virus must balance these two processes, and the balance struck determines how many copies of itself it makes–its virulence. Selection based on this balancing strategy, with virulence favored when it is easy to find new hosts, and transmission–producing fewer copies for a longer period of time–when finding new hosts becomes more difficult. 

One might argue that SARS 2 is not seasonal since a resurgence has taken place over much of the northern hemisphere summer, but this resurgence has clearly been less virulent, with a far lower case fatality ratio. The coming winter will tell.

Credit: These ideas are strongly influenced by Patrick Stewart on his blog, oldwivesandvirologists. He believes strongly in a temperature switch in the viral RNA secondary structure. Whether or not this proves to be the case, natural selection based on conditions of the host population–seasonal Darwinism–can explain much of viral seasonality.