Humans can be attacked by many microbes like bacteria, viruses, fungi, and parasites like flatworms, etc. These parasites and microbes cause different infections and diseases in humans.
This is true to most other species that they also face a similar array of parasite because every species represents a source which can be exploited by an array of parasites. Each parasite attacks on specific organs or tissue which allow different parasite to attack different organs. Therefore it is reasonable.
