system
Given the following post, respond as if you're commenting on Reddit:
human
What is the reason that virus variants only very briefly coexist?

How can it be explained that virus variants such as the alpha variant or delta variants are no longer existent, apparently being replaced by new variants. Why are there variants only briefly in parallel?
gpt
Exponential growth. Modest differences in transmissibility allow one variant to out-spread another.

Let's start with two people. One has Variant A, and one has the newer, slightly-more infectious Variant B. If Variant A infections two people for every original infection, and Variant B infects 3, then after a dozen cycles there will be more than 100 times as many cases of B as there are of A.

As well, previous infection may protect against future reinfection by the same variant. (Whereas previous infection may or may not provide protection - or provide less robust protection - from infection by a variant.) If Variant A has burned through a population already, there will be more people susceptible to infection by B.