For almost 1.5 year running colony no one get sick for Malaria, Flu, gut worms, etc or have wound infection. Maybe I just too lucky, but I also try to generate Disease Incident in developer mode. It possible to infect pawns with Mechanites, or muscule parasites, but still not possible to infect with Flu or Malaria (i not try the whole list of diseases). No one is affected by penoxyciline, to be clear. Not sure about rare diseases like carcinoma.