Parasites That Live in Humans

Parasites live by getting food from a host, which harms the host. Parasitic infections can occur in plants, animals and humans. The Centers for Disease Control (CDC) and the Mayo Clinic have identified several parasites that commonly afflict humans, and their symptoms.

Cryptosporidium

• Cryptosporidium, commonly referred to as "crypto", is a parasite that usually infects people through contaminated water. Crypto can survive chlorination and can infect people drinking tap water, and can also live in lakes, streams and rivers where people swim or play. People with crypto may have symptoms including nausea, vomiting, diarrhea, cramps and fever that can last up to 30 days; crypto can be life threatening for people with immune system disorders.

Giardia

• Giardia intestinalis is spread by contaminated water, food and contact with feces. People who swim in local creeks, rivers and lakes can become infected; changing diapers without washing hands and having anal sex can also spread giardia parasites. People infected with giardiasis may have symptoms such as nausea; frequent diarrhea that has a foul odor; and significant weight loss.

Roundworms

• Many species of roundworms, which are also referred to as helminths, live in the intestines of humans and cause illness.
  Pinworms are the most common type of roundworm infection, and are spread by dirty hands or walking barefoot on contaminated dirt; the main symptom is intense itching around the anus.
  Ascaris worms can be up to 13 inches long and their life cycle includes stages in the human lungs and intestines. This infection is called ascariasis. Symptoms include vomiting, abdominal pain and sleep problems.
  Hookworms infect humans who walk on dirt with bare feet, and cause skin problems, abdominal pain, colic and diarrhea.

Toxoplasma

• Toxoplasma gondii parasites infect people who eat contaminated food or who come into contact with feces of infected cats. According to the Centers for Disease Control, up to 60 million Americans have this infection, although most will not have any symptoms or illness. However, pregnant women, babies and people with immune system problems can develop severe complications including brain damage and loss of vision.

Trichinella

• The trichinella spiralis parasite infects humans as well as most other mammals. People who eat contaminated meat that is undercooked can become infected, and the parasite forms a cyst that opens in the stomach and can cause stomach cramps, vomiting and diarrhea. After leaving the stomach, the parasites invade the muscles and can cause damage to the joints as well as muscle pain.