Roger M. Knutson, Fearsome Fauna: A Field Guide to the Creatures That Live In You

Types of Parasites

There are three main types of parasites that can affect humans: protozoa, helminths, and ectoparasites. They can live on or in the human body and cause various diseases. The Cleveland Clinic provides clear, succinct descriptions, as follows.


Ectoparasites live on the outside (exterior) of the body. They carry diseases between other animals and humans and usually carry infections through blood. They generally include the following:

  • Fleas are small, wingless insects with strong back legs that they use to jump long distances. Infected fleas can spread disease when they bite or if a person accidentally swallows an infected flea.
  • Head lice and pubic lice (crabs) are tiny, flat insects that travel by crawling. Both types of lice travel from person to person through close contact, which may include sexual intercourse or sharing personal items like hats, sheets, pillows, or towels.
  • Mites are small arachnids, relatives of spiders and ticks. They’re small, about as tall as a stack of 10 sheets of paper. They may cause scabies.
  • Ticks also are arachnids. Their bites usually don’t cause pain or itchiness. They typically bite you and then burrow into your skin.


Helminths are worms that usually live in your gastrointestinal (GI) tract. They are visible to the naked eye in their adult stage — and range from greater than 1 millimeter to a little longer than 39 inches (slightly smaller than the width of a doorway).

Ascariasis, a soil-transmitted parasite

The main types of helminths that affect people include the following:

  • Flukes (Trematodes) are a type of flatworm that can spread through contaminated water or aquatic animals (including snails, crabs, and fish). The many different types of flukes may infect your blood, urinary bladder, liver, lungs, intestines, and other organs.
  • Tapeworm (Cestode) adults are long, flat worms that live in the intestines and feed on the nutrients that you get from eating food. They spread by laying eggs that leave your body when you poop. The eggs then spread through infected food and water or undercooked meat.
  • Roundworms (Nematodes) are small parasites that also live in your intestines. They spread from infected poop or soil.


Protozoans are one-celled organisms that live in your intestines or blood and tissues. You can’t see them without a microscope. They can spread through several means, including contaminated food or water and person-to-person contact.

A variety of protozoan parasites found in drinking water

There are tens of thousands of types of protozoans. Experts classify them according to how they move. The main types that affect people include the following:

  • 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 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) aren’t capable of moving in their adult stage. They eat the food you are digesting or your body fluids. The sporozoan Plasmodium causes malaria, and Cryptosporidium causes cryptosporidiosis.

Human(-Infesting) Parasites

According to Knutson, the parasites that live in humans are generally ugly, not smart, and extremely motivated to reproduce.

Given where they live, parasites have little need for such sense organs as eyes or a sense of smell. As Knutson says, “Better you should have a good set of hooks, suckers, or clamps or a mouth for hanging on or the capacity to swim with vigor.”

According to the CDC, more than 60 million people in the US are infected with parasites. The overall prevalence of intestinal parasitic infections was 16.1%, according to the NIH. Soil-transmitted helminth infections (14.3%) were more common than protozoan infections (1.8%).

I’ve focused on the U.S., where neglected parasitic infections are not rare. NIH says neglected parasitic infections affect at least 12 million Americans, either through new infections (e.g., trichomoniasis) or from prevalent persistent infections resulting in chronic conditions. Limited access to clean water and sanitation are major factors. Practicing good hygiene, thoroughly cooking meat, and drinking clean water also helps prevent many parasites.

Roger M. Knutson, Fearsome Fauna: A Field Guide to the Creatures That Live In You

Treating Parasites

Thanks to the coordinated efforts of many international organizations, including Jimmy Carter’s legacy Carter Center, guinea worms (a type of nematode parasite) have been almost completely eradicated. In 1986, doctors estimated 3.5 million people had guinea worm infections. In 2023, there were only 14 cases reported worldwide.

Treatment is extremely important. You may develop a serious infection with severe symptoms if you don’t get treatment and follow your doctor’s orders carefully.

Treatment specifics depend on what type of parasite you have.

For treating ectoparasites such as lice, fleas, and ticks:

  • Bathing with soap
  • Shampoos
  • Ointments
  • Washing your clothing, bedding, and towels in hot water
  • Vacuuming carpets, mattresses, and furniture and emptying the vacuum bag into the trash outside

For other types of parasites, your healthcare provider may prescribe:

With a proper diagnosis and treatment, most people make a full recovery. So carefully follow your provider’s instructions. If you don’t, your parasite may come back.

Symptoms of Parasites

How can you know? Because there are so many different types of parasites, the symptoms also vary widely. According to the Cleveland Clinic, common parasite symptoms include:

Not too long ago, some people actively sought the weight-loss symptoms of parasites!
  • Diarrhea
  • Nausea and vomiting
  • Abdominal pain
  • Unexplained weight loss
  • Increased appetite
  • Muscle aches
  • Fever
  • Chills
  • Problems sleeping (insomnia)
  • Fatigue
  • Weakness
  • Skin rash

You may have a parasite and no symptoms, or the symptoms may appear a long time after infection. You may also not have any symptoms and accidentally pass a parasite to another person who develops symptoms.

Parasites Everywhere!

NIH estimates that there are between 75,000 and 300,000 helminth species alone parasitizing vertebrates. But they have no credible way of estimating how many parasitic protozoa, fungi, bacteria, and viruses exist. According to an article in Salon, several estimates suggest that parasites actually outnumber “free living” organisms by about 3 to 2. In short, they’re everywhere.

Conclusion: although you might feel queasy to learn that you have a parasitic infection, don’t take it personally. Instead take appropriate steps to recover and to avoid having anything eating you in the future!

Bottom Line: Parasites are an ugly fact of life. But you don’t have to despair. And learning about them can be fascinating.

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