When normal animal gets sick, it dies. The body dies, so the viruses in the body can’t go anywhere and die also. Disease not spread

When bats get sick, it doesn’t hurt them. They survive and the virus in them gets to grow and mutate and eventually evolve to jump species and infect humans because the bat is alive flying around humans or being ate by humans

Where other animals would get sick and die, ending the viruses spread, bats easily survive virus’s thus are able to incubate them and spread them around