Basicly everything gets a disease. Did I start something in one of the other topics? Like that one with "How Long can Yautjas Live?" and then in one post I said "...there are diseases to overcome."
But yeah really. Everything can get sick. You can take a fully grown Xeno, a natural killing machine because it was made to do so, but if you take a human disease that is deadly. It can kill that poor creature.
Edit: But there is also an immunity system in everything. It'll help fight back harmful bacteria in the body. That is why we live our short dull lives on earth longer.
Look around each corner before you move because the chances are, you are not gonna make it.