1. Ascaris: the Ascaris lumbricoides roundworm causes a disease called ascariasis. It usually affects children, who eat or drink food or water that are contaminated with the worm’s eggs. Most people with ascariasis do not have symptoms unless there are a great many worms in the system.
2. Strongyloides: This parasite is also a roundworm and mostly affects people who live in countries that have tropical or semi-tropical climates. Like ascariasis, it is often asymptomatic. This is called uncomplicated disease. Disseminated strongyloidiasis is a risk for patients with weakened immune systems and can be fatal.
3. Trichiuris: This is a whipworm that parasitizes the human intestines and causes trichuriasis. Its long, whiplike shape gives it its name. People can become infected when they eat the eggs. Using night soil puts people at risk for trichuriasis, because the eggs mature in the soil. It is important to wash the hands and the produce if night soil has been used as a fertilizer.
4. Ancylostoma: The ancylostoma is a species of hookworm, which gets its name from the teeth at its head. It lives in the small intestine and enters the body through the skin, usually the feet. It enters the bloodstream, swims to the lungs and is coughed up and swallowed. This is how it enters the intestines. Ancylostoma duodenale is found worldwide. Another type of hookworm, Necator americanus, has a similar life cycle.
5. Giardia: This parasite, specifically Giardia lamblia, is responsible for the common infection giardiasis. People and even pets become infected through contact with contaminated soil, food and water. It can also be sexually transmitted. The parasite can live outside of a host for a long time, and it is found all over the world.
6. Entamoeba: This is a genus of amoeba that can be parasitic. Amoebae are protozoans. Entamoeba histolytica is responsible for a potentially serious infection called amoebiasis, which can cause abscesses in the liver and amoebic dysentery. Entamoebae usually live in the intestines, but some live in the mouth.
7. Cyclospora: Cyclospora is another one celled protozoan that causes a disease called cyclosporiasis. Some people have no symptoms, but others have watery, explosive diarrhea. The diarrhea alternates with constipation. Cyclosporiasis is contracted through ingesting contaminated food and water. Eating fresh, unwashed produce puts people at risk for the infection.
8. Cryptosporidium: This common infection, nicknamed “crypto,” is caused by a protozoan. It can parasitize other animals as well as humans and causes watery diarrhea, cramps, dehydration, nausea and vomiting. As with many other parasites, it is obtained through eating and drinking contaminated food and water.
9. Fasciolopsis: This is a fluke, which is a type of worm or helminth that causes fasciolopsiasis. This infection occurs mostly in Asia. Fasciolopsis buski is notable because it is very large. It can grow to as long as 3 inches. It is also notable because it lives in the intestines and not in the liver, as other parasitic flukes do.
10. Cystoisospora: This parasite is a coccidian, which means it is a single celled organism that parasitizes the intestines of animals. It is spread through eating contaminated food and drinking contaminated water. People with weakened immune systems are especially at risk.
11. Tapeworm: Tapeworms are famous for their great size and long life. A tapeworm, which is a type of flatworm, can be 50 feet long and live in the gut for as long as 30 years. People are at risk for getting a tapeworm through eating undercooked or raw meat. However, a tapeworm infection often doesn’t cause symptoms, though segments may break off now and then. Sometimes they are seen in the feces.