The simplest and the diseases they cause


Protozoa are single-celled organisms classified as eukaryotes (organisms whose cells contain membrane-bound organelles and nuclei). Other eukaryotes include:

  • People
  • Other animals
  • Plants
  • Seaweed
  • Helminths
  • Mushrooms
Paul Bradbury / Getty Images

The simplest are found everywhere. They can live by themselves as free-living organisms in their environment, often in soil, water, or moss. They can also be inactive cysts, allowing them to survive dry times.

Some of the simplest parasites . Others live in symbiosis with other organisms, each of which depends on the other for its survival.

What's in a name?

The word protozoa comes from the word protos, which in Greek means "first" and zoia , which means "animal". It was first thought of in the 19th century. Before this, microscopic protozoa, defined by their organelles, could not be fully studied.

The simplest

The simplest are divided into different classes:

  • Sporozoa (intracellular parasites)
  • Flagellates (with tail-like structures that wave to move)
  • Amoebas (which move through temporary projections of the cell body, called pseudopods).
  • Ciliates (which move by affecting various hair-like structures called cilia)

Protozoal infections can be transmitted through ingestion of cysts (dormant state), sexually, or by insect vectors.

Many common, and not very common, infections are caused by protozoa. Some of these infections cause illness in millions of people each year; other infections are rare and can go away.

The simplest diseases

Common infectious diseases caused by protozoa include:

These infections occur in many different parts of the body. Malaria infections start in the blood , giardia in the intestines, and toxoplasmosis in the lymph nodes, eyes, and (worryingly) the brain.

Other protozoan diseases include:

  • African trypanosomiasis (sleeping sickness) : caused by Trypanosoma brucei gambiense (98% of cases). and Trypanosoma brucei rhodesiense (2%). Both are transmitted by tsetse fly bites .
  • Amoebic dysentery : due to Entamoeba histolytica), which causes diarrhea and gastrointestinal upset. It can also cross the intestinal wall and enter the bloodstream and other organs such as the liver, where it can form liver abscesses.

Can sleeping sickness be eradicated?

Sleeping sickness flies are found in at least 36 countries around the world. The disease causes severe neurological effects and treatment is difficult. In poorer areas with limited resources, it is difficult to identify and treat.

Most of the cases occur in the Democratic Republic of the Congo, where plans are underway to dramatically reduce the spread of the disease and its burden, and perhaps even to extinguish these protozoa.

Watch out

Treatment options depend on the protozoa that are infecting you. Some of them are much more successful than others.

Malaria is a global disease that is easily treated , although treatment depends on the type of malaria ( Plasmodium falciparum , Plasmodium knowlesi , Plasmodium malariae , Plasmodium ovale , and Plasmodium vivax ).

Treatment also depends on whether the protozoa are drug resistant. During the last decades, P. falciparum has become especially resistant to some important drugs .

Infection detection

Unlike other pathogens, protozoan samples cannot be identified simply by culture. They can sometimes be seen under the microscope inside red blood cells (as in malaria) or in feces (as in lamblia and E. histolytica ).

There are also rapid blood tests for antibodies or antigens, as well as PCR tests that determine your genetic material .


Toxoplasmosis can be identified in a number of ways depending on where it is causing the infection, including:

  • Blood test for antibodies.
  • PCR tests
  • Special fabric stains
  • Direct isolation of the exciter


Giardia can be detected by examining the stool for antigens and looking at the stool under a microscope. Multiple stool samples (possibly three) may be required for diagnosis .

Entamoeba histolytica

E. histolytica can also be identified from stool samples such as lamblia. It can be identified under a microscope, by a PCR test, an antigen test, or a blood antibody test.

Human African trypanosomiasis

Human African trypanosomiasis can be diagnosed with blood tests, fluid tests, or a lymph node biopsy (or chancre wound).

Trypanosoma brucei rhodesiense

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

Trypanosoma brucei gambiense

T. b. gambiense has a lower protozoan count in the blood, so blood microscopy usually cannot identify it, but microscopic examination of a lymph node (posterior lymph node) biopsy is more likely to identify an infection .

Frequently asked questions

  • The simplest are microscopic single-celled organisms. Protozoa can reproduce in humans and pass from one person to another. They can cause infectious parasitic diseases such as malaria, giardia, and toxoplasmosis.

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

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