Comment by Kliment

I have some limited background in this though it's not my main area of expertise.

The vast majority of non-bacterial species that are subjected to severe evolutionary pressure go extinct. The main reason this happens is they cannot adapt rapidly enough to respond to evolutionary pressure.

Malaria is a very complex parasitic disease which depends on a whole series of things going right across multiple hosts. If it's disrupted in either the mammal host or the insect host it's unable to reproduce. There's a number of disruptive events that can happen (such as blood cells being the wrong shape, preventing infection from the parasite). There's no variants of malaria that can infect sickle-shaped blood cells even though sickle-cell anemia is very common in malaria regions.

Unlike viruses and bacteria, there's no easy mechanism for a new variant of a parasite to spread its mutation laterally. The replication mechanism for parasites occurs much less frequently and produces much fewer copies than bacteria or viruses do. RNA-based viruses have very high mutation rates in general, due to the defects in RNA replication. The viral life cycle happens anew with every cell infection. Bacteria are even more adaptive - they can actually exchange genetic material even between replications, and of course replicate exponentially without depending on host cells. This is why viruses and especially bacteria can rapidly respond to evolutionary pressure. Other species are not so lucky. For a malaria parasite to respond to evolutionary pressure, it would have to go through its entire multi-host song and dance on every replication cycle. This already massively slows down mutations, as the number of mutations depends on the number of replications and the probability of mistranscription in each replication. But the important thing is that we are already using heavy interventions to prevent the spread of malaria - if we can drop the base rate of spread by some tens of percent and keep those measures, malaria will go extinct. If we remove the index host species (the particular susceptible mosquitoes) it's going to take a very long time for the parasite to successfully infect another species, as the number of replications drops massively and there's no replication reservoir where a new variant can develop. As the individual parasite's lifecycle is short, extinction is much more likely than adaptation.