Epidemics are a relatively new phenomenon as far as human evolution is concerned. Within a purely human set of hosts, germs don't feel evolutionary pressure to

Epidemics are a relatively new phenomenon as far as human evolution is concerned. Within a purely human set of hosts, germs don't feel evolutionary pressure to become more deadly. If anything, it's advantageous for a germ to be less deadly. The most successful viruses are the common cold viruses, not SARS or Ebola. Epidemics only really became a thing once we started domesticating animals and therefore being around live animals for prolonged periods of time. This allowed germs originally adapted to these animals the opportunity to jump over to human hosts, for which they may be more dangerous than for the original host animal. For more details on this point, I can recommend this video on why the native Americans were killed by diseases imported by the European conquerors, but the conquerors didn't bring any deadly diseases back to Europe: https://www.youtube.com/watch?v=JEYh5WACqEk https://www.youtube.com/watch?v=JEYh5WACqEk