Actually there is a strong theory that humans started having sex/reproduce to ensure survival from parasites. Go to Africa ppl are very badly infected by them yet they reproduce with no issues. Your immune system by now adapted to almost any parasite once again you might feel horrible but you will stay alive say thank you for that. What? I don't know what I'm saying. Okay. First go and look deep into parasitology and immunology before you say anything else. Now some parasites do kill but it's more rare. Most of them avoid it since killing the host will kill them.