I just finished my first year of medical school. Our last quarter was called Host and Defense and covered microbiology, immunology and infectious diseases. It was probably my favorite block so far because now I can watch House and guess the ending.
The coolest parasite I read about is the schistosome, which causes schistosomiasis. The worms penetrate your skin, travel through your bloodstream, and a guy worm and a girl worm get together in the portal vein and mate. This is not a transitory love hug, though. They remain in the copulatory embrace for the rest of their lives. Isn't that bizarre?