Well, I meant https://en.wikipedia.org/wiki/Evolutionary_pressure
Think of a population as holding at any given time a number of individuals carrying mutations. The fringe mutations, such as being able to be hosted by a different type of mosquito (say, better survivability in the host) don't provide much of a benefit to survival. But if the default host population goes away, those mutations become super valuable and will be selected.
See https://www.scientificamerican.com/article/transgenic-mosqui... for a discussion on this topic.
What does evolution “pressuring” mean? Either the organism (or virus) has sufficient chances to randomly mutate a a trait that enables it to survive, or it goes extinct.