Look into viral theory, it may provode an answer.
Imagine a non-life threateneing virus which spreads easily. This is an incredibly successful virus because it promotes its own survival by not killing the host. Now imagine a similar virus which kills at an increased rate. This new virus must be able to transmit faster between hosts or it will die out.
Corona viruses (of which strains of the "common cold" are frequently attributed) are the first type of virus. Symptoms are mild, so people unwittingly transmit it. Any mutations which are more lethal quickly die out because hosts exhibit greater symptoms and are either quarantined by the surrounding population or they die quickly.
Now imagine a treatment has been developed that proactively suppresses symptoms of this more lethal strain. Those treated will exhibit symptoms more akin to a less-lethal strain, and the more-lethal strain will continue to flourish.
Fast forward several viral generations and you may end up with incredibly lethal strains which still retain the ability to quickly pass through populations. The treated population may continue to be "safe" against these strains-if treatment development and dispersal keeps up.
Now you can use conjecture to imagine potential drawbacks of this situation.
(post is archived)