There are lots of tiny organisms in the world, probably more than humans. So the earth would have its fair share of things that would make your skin crawl. And then there are some that can fail vital organs, and some of these are parasites. I thought because we studied the hydra and planaria it would be fun to give people nightm-- I mean do research on this topic. An interesting parasite is the threadworm (Strongyloides) and an other organism, the water bear.

The threadworm most commonly infects its host by larvae infected soil. Some symptoms are: abdominal pain, bloating, heartburn, intermittent episodes of diarrhea and constipation, a dry cough, and rashes. Rarely people will develop arthritis, kidney problems, and heart conditions. Click here to read more about this parasite.

The water bear is not exactly invincible or a parasite but probably can survive the end of the world.This is the water bear (tardigrades). The water bear is a microscopic organism that can survive the extreme heat and cold, radiation, and the vacuum of space. And to top it all off, they live for 200 years... So expect to (not) see this creature around. How studies were done is they sent the FOTON-M3 (a rocket) to space with some water bears and 68 percent of the bears lived! Also some water bears have been found under a rock where there hasn't been any water in 25 years. So, what gives you the heebie jeebies? (by Griffin)