breath? yes, they do.
ok, well, if I was explaining it to my own sons (5 and 9) I'd say "A parasite is an animal that takes it's food, drink and air from another animal. You have food and drink in your blood and in your tummy (Stomach) which the parasite can get at. You have air in your tummy too which the parasite can breath." You and I both know it's more complicated than that but that's all I'd say to keep him happy until he's a bit older and can understand more. If he asks if it makes people sick I'd say yes because it takes food and energy from the people when it eats so we give them pills to get rid of the parasites. Hope that helps, if you want more details I can give you some links to related sites to read in your own time (or I'm sure you can do that yourself of course). :)