Oh WOW! I could write a book on this question. I'll give you the short answer here - there are hundreds of parasites that can live in the human body. 70% are microscopic and 30% are visible (the worms). Within these groups, they all reproduce differently with most microscopic ones reproducing very very rapidly - within HOURS. The "worms" reproduce more slowly and usually cycle within 21 days. I hope this is what you need, if not, we'll have another go at it :)