What are parasites?
Parasites are organisms that live in, on or with another organism (host). They feed, grow or multiply in a way that harms their host. However, they need their host for their survival. For this reason, they rarely kill their host, but they often carry diseases that can be life-threatening.
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What do parasites do to your body?
Parasites feed, grow or multiply in a way that harms your body.
What are the three types of parasites?
The three main types of parasites that cause disease in people include:
Ectoparasites
An ectoparasite is a parasite that lives on the outside (exterior) of its host. They’re vectors (living things that carry diseases between animals and humans) that usually carry infections through blood. Many creatures that healthcare providers classify as vectors feed on your blood. They generally include:
- Fleas. Fleas are small, wingless insects with strong back legs that they use to jump long distances. Infected fleas can spread disease when they bite their host or if a host accidentally swallows an infected flea.
- Head lice and pubic lice (crabs). Lice are tiny, flat insects that travel by crawling. Head lice live on the hair on your head. Pubic lice live in your pubic hair, near your genitals. Both types of lice travel from person to person through close contact, which may include sexual intercourse or sharing personal items like sheets, pillows or towels.
- Mites. Mites are small arachnids (eight-legged arthropods) that are relatives of spiders and ticks. They’re smaller than 1 millimeter (0.04 inches, about as tall as a stack of 10 sheets of paper). Certain types may cause scabies.
- Ticks. Ticks are arachnids. Their bites usually don’t cause pain or itchiness. They typically bite you and then burrow into your skin. They’re common in wooded areas or grassy fields.
Helminths
Helminths are parasitic worms that usually live in your gastrointestinal (GI) tract. Your GI tract is a series of hollow organs that connect to each other from your mouth to your anus, including your stomach, small intestine and large intestine. Helminths are visible to the naked eye in their adult stage — they range from greater than 1 millimeter to greater than 1 meter (a little longer than 39 inches, which is slightly smaller than the width of a doorway).
The main types of helminths that affect people include:
- Flukes (Trematodes). Flukes are a type of flatworm. They can spread through contaminated water or aquatic animals, including snails, crabs and fish. There are many different types of flukes, and they may infect your blood, urinary bladder, liver, lungs, intestines and other organs.
- Tapeworms (Cestodes). Adult tapeworms are long, flat worms that live in the intestines. They feed off the nutrients that their host gets from eating food. They spread by laying eggs in their host’s body. The eggs then leave the host’s body when the host poops. The eggs spread through infected food and water or undercooked meat.
- Roundworms (Nematodes). Roundworms are small parasites that live in your intestines. They spread from infected poop or soil. There are many different types of roundworms.
Protozoans
Protozoans are one-celled organisms. You can’t see them without a microscope. They may live in your intestines or blood and tissues. They may spread through contaminated food or water, person-to-person contact or through the bite of a vector.
There are tens of thousands of different types of protozoans. Experts classify them according to how they move. The main types that affect people include:
- Amoeba. Amoebas form temporary “false feet” (pseudopods) to move. The amoeba Entamoeba histolytica (E. histolytica) causes dysentery.
- Ciliates. Ciliates use many short, hairlike structures (cilia) to move and gather food. Balantidium coli (B. coli) is the only ciliate that affects people. It causes dysentery.
- Flagellates. Flagellates use one or many whip-like structures (flagella) to move and sense their surroundings. The flagellate Giardia intestinalis causes giardiasis, and Trypanosoma brucei causes sleeping sickness.
- Sporozoans (apicomplexan). In their adult stage, sporozoans aren’t capable of moving. They eat the food their host is digesting or their host’s body fluids. The sporozoan Plasmodium causes malaria, and Cryptosporidium causes cryptosporidiosis.
How common are parasitic infections?
Parasites and parasitic infections are common. They affect millions of people throughout the entire world. Many people may not notice they have an infection because they have few symptoms. Others may have serious illnesses.
