> Evolution isn't random. Mutations are random. Natural selection is not. There's a difference. Animals, bacteria, and viruses do not direct their evolution. There's no, "hey, there's a medicine fighting us so we had better develop some resistance." Mutations are going to happen regardless of vaccines or medicine. What changes is the natural selection that follows. The mutation for more deadly strains of Marek's disease would have happened without any vaccine or medicine. Think of it like an air filter. If you measure the particulates in the air before running and air filter and after, you will find that the particle sizes are much smaller after. It would be a logical fallacy to say the filter caused particles to get smaller. All the filter did was remove the larger particles, leaving the smaller ones that were already there the whole time.