Some parasites enter through contaminated water or food. Example: Cryptosporidium - protozoan parasite that infects the intestines of humans and animals. Have you ever gotten sick after traveling or drinking untreated water?
On a mission trip to El Salvador I picked up some type of bug. I couldn't directly attribute it to the water as I was very careful not to drink anything that wasn't bottled BUT, I haven't ever been so sick. It was horrible!