There are also parasites that live on the skin surface, like mites. Dog mites affect the skin and they move from animal host to human host and both may be affected. Mites cause skin problems to pets and their owners.

Tape worms
These are unusual parasites. While bad parasites may claim human lives or animal lives, these parasites or parasitic worms may actually help cure colitis, hay fever, asthma and other auto-immune diseases.
The beneficial parasites are gut-worms or tape worms. Many backpackers in Asia believed that they are able to ward off diseases and bad parasites by having gut worms in their system.

Tapeworms can be helpful to adult individuals. However, if tapeworms are not controlled in younger children, they will get malnutrition. Also, long term tape worm infection may cause brain, eye and other tissues diseases.

Wolbachia
Wolbachia are parasite-like microbes. They are in some insects. They may cause female hosts to be less fertile. However in larger quantities some studies show that this same parasite may evolve to be helpful instead of being harmful. Wolbachia sometimes boost a host's fertility by helping the cells breath oxygen better.

Gammarus Roeseli
Gammarus Roeseli are fresh water amphipods. These parasites help a host become tolerant to salinity in the water, helping them become resistant to waterborne diseases. These parasites may be beneficial to the body however in large quantities and long term infection diseases may be developed.

Scientists are trying to engineer more beneficial parasites. For example, they are studying some parasites help the body fight intracellular pathogens like viruses and bacteria. This helps the host survive, which is good for the parasite. Thus, if the body responds to viruses and bacteria causing inflammation, the parasitic worms mount an anti-inflammatory defense. The greater worm burden, the greater the anti-inflammatory effect it can build up. This is weird.

In this study, patients suffering from Crohn's disease or ulcerative colitis are asked to drink a concoction of 2,500 whipworm eggs mixed with Gatorade. After this, the infection became short term and there was significant reduction in the Intestinal Bowel Disease symptoms.

Not all parasites are completely harmful, and it is always a good idea to see a doctor if you are concerned. You are probably better off without them but some may be good for you in small amounts.