Protozoa and the Illnesses They Cause

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Protozoa are a group of 65,000 (or possibly more) single-cell organisms. They are part of what's called the eukarya domain and include the cells that make up humans, animals, plants, fungi, and parasites.

Most humans will "host" a protozoa in or on their body at some point in their life. While not all protozoans found in the environment are dangerous, some cause diseases like malaria and giardia, both of which can lead to diarrhea.

A man with a stomachache and a headache
Paul Bradbury / Getty Images

This article explains the role of protozoa in infectious disease and how protozoan diseases are treated.

What Are Protozoa?

Protozoa are broken down into different classes:

  • Sporozoa (intracellular parasites)
  • Flagellates (with tail-like structures that flap around to move them)
  • Amoeba (which move using temporary cell body projections called pseudopods)
  • Ciliates (which move by beating multiple hair-like structures called cilia)

Infections caused by protozoa can spread through ingesting cysts (the dormant life stage), sexual transmission, or through insect vectors (insects that transmit diseases through bites or stings).

Protozoa cause some common and some uncommon infections. Some of these infections cause illness in millions of people each year; other diseases are rare.

What's In a Name?

The word protozoa comes from the word protos, Greek for "first," and zoia, which meant "animal." It was first coined in the 1800s. Before then, the microscopic protozoa, defined by their organelles, could not be fully appreciated.

Protozoan Diseases

Common infectious diseases caused by protozoans include:

These infections arise in very different parts of the body. For example, malaria infections start in the blood, giardia begins in the gut, and toxoplasmosis can infect lymph nodes, the eye, and the brain.

Less common protozoan diseases include African trypanosomiasis and amoebic dysentery.

African Trypanosomiasis

African trypanosomiasis, also called "sleeping sickness," is caused by Trypanosoma brucei gambiense (98% of cases) and Trypanosoma brucei rhodesiense (2%). Tsetse fly bites spread both.

The flies that spread sleeping sickness live in at least 36 countries. The disease causes serious neurologic effects, and the treatment is complex. In poorer, resource-limited areas, it's hard to identify and treat.

Most cases occur in the Democratic Republic of the Congo, where people are working to reduce the spread of the disease and its burden—and possibly even drive these protozoa into extinction.

Amoebic Dysentery

Amoebic dysentery is due to Entamoeba histolytica, which causes diarrhea and gastrointestinal upset. It can also travel through the walls of the intestines and go into the bloodstream and other organs, like the liver, where it can create liver abscesses.


Common protozoan infectious diseases include malaria, giardia, and toxoplasmosis. Less common diseases include African trypanosomiasis and Amoebic dysentery. Each condition affects the body differently.

Detecting Infections

Unlike other pathogens, cultures do not identify protozoa. However, sometimes you can see them under a microscope inside red blood cells (as in malaria) or in the stool (as in giardia and E. histolytica).

In addition, rapid blood tests for antibodies or antigens and PCR tests can detect their genetic material.


Healthcare providers can identify toxoplasmosis in several different ways depending on where it's causing an infection, including:

  • Antibody blood tests
  • PCR tests
  • Special stains of tissue
  • Direct isolation of the pathogen


Giardia can be found through a stool antigen test and by looking at stool under a microscope. It may take multiple stool samples (maybe three) to diagnose this.

Entamoeba Histolytica 

E. histolytica can also be identified from stool samples. It may also be identified under a microscope, through a PCR test, antigen test, or an antibody test of the blood.

Human African Trypanosomiasis

Diagnosing human African trypanosomiasis involves blood tests, fluid tests, or a biopsy from a lymph node (or a chancre wound). 

Trypanosoma Brucei Rhodesiense

T. b. rhodesiense parasites can usually be found in the blood of infected people. 

Trypanosoma Brucei Gambiense

T. b. gambiense has a lower burden of protozoa in the blood, so blood microscopy is usually unable to identify it. However, microscopic examination of a lymph node biopsy is more likely to identify the infection. 


Diagnosing protozoan diseases may involve blood tests, stool tests, or biopsies. A healthcare provider will decide which test is appropriate based on which protozoa they suspect.


Treatment options depend on what protozoa are infecting you. Some are a lot more successful than others.

For example, malaria is a common illness worldwide that has straightforward treatment. However, the treatment depends on the type of malaria (Plasmodium falciparumPlasmodium knowlesi, Plasmodium malariae, Plasmodium ovale, and Plasmodium vivax).

Treatment also depends on whether the protozoa are drug-resistant. P. falciparum especially has grown resistant to some essential drugs over the last few decades.


Protozoa are single-celled organisms that can sometimes cause diseases. Common protozoan diseases include malaria, giardia, and toxoplasmosis. Diagnosing protozoan illness may involve blood tests, stool tests, or biopsies, depending on which protozoa a healthcare provider suspects. Treatment varies based on the cause.

Frequently Asked Questions

  • What are protozoa?

    Protozoa are microscopic, single-celled organisms. Protozoa can multiply in humans and transmit from one person to another. They can cause parasitic infectious diseases like malaria, giardia, and toxoplasmosis.

  • What are the types of protozoa?

    There are four types or classes of protozoa. These include the sporozoa (intracellular parasites), flagellates (which use a tail-like structure to move), amoebas (which move using pseudopods or a temporary cell body projection), and ciliates (which move using hair-like structures called cilia).

10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Ingrid Koo, PhD
 Ingrid Koo, PhD, is a medical and science writer who specializes in clinical trial reporting