Toxoplasmosis

What toxoplasmosis is, and who needs to care

A third of humanity carries this parasite, and for almost all of them nothing ever happens. This page explains what it is, how people catch it, and the few situations in which it deserves real attention.

By Jeroen Saeij · Updated 17 September 2026

A parasite with a strange life

Toxoplasma gondii is a single-celled organism, far too small to see, that can infect almost any warm-blooded animal: mice, sheep, pigs, birds, sea otters, and us. Estimates put the number of infected people at up to a third of the world’s population¹. How common infection is depends on where you live and what you eat. In the United States about one person in ten carries antibodies to the parasite², which the CDC translates into more than 40 million people³. Parts of Latin America, Eastern Europe, the Middle East, South-East Asia and Africa have far higher rates⁴.

The parasite can multiply in any of these hosts, but it can only complete its sexual cycle in the gut of cats and other felines. An infected cat sheds millions of microscopic egg-like oocysts in its droppings for a week or two, and after that it is largely immune to shedding again⁵. Those oocysts are extraordinarily tough. They survive in soil and water for months, and my own lab recently found part of the reason: a family of proteins that protects them against drying, freezing and salt⁶.

Every other animal is a stopover. After the parasite is swallowed, it spreads through the body for a few weeks, multiplying rapidly inside cells. Then the immune system brings it under control, and the survivors retreat into dormant cysts in muscle and brain. There they stay for life, mostly silent, waiting for a cat to eat the animal and start the cycle again.

How people catch it

People are stopover hosts too, and we pick up the parasite in the same ways other animals do³:

• Meat. Eating meat that contains cysts and has not been cooked or frozen enough to kill them. In a large European study this was the main route, responsible for between a third and two thirds of infections in pregnant women, depending on the city⁷. Undercooked lamb, pork, game and locally made cured meats stand out⁷⁸.
• Oocysts from the environment. Soil on unwashed vegetables or hands, untreated water, and raw shellfish that have filtered oocysts out of contaminated water⁸⁹. In North America this route is more important than most people assume: among mothers of congenitally infected babies in one large study, 78 percent had been infected by oocysts, and half of them could not name any risk they had taken¹⁰.
• From mother to unborn child. Only when the mother catches the parasite for the first time during pregnancy, or just before it¹¹.
• Rarely, through an organ transplant or blood transfusion³.

Notice what is not on this list: stroking a cat. Direct contact with cats is not thought to be a major source of human infection, because cats shed oocysts only briefly, the oocysts are not infectious when fresh, and cats keep themselves clean¹². The cats page goes into this.

What infection feels like

For a healthy adult, usually nothing. Some people have a few weeks of swollen glands in the neck and a tired, feverish feeling that passes for a mild flu¹. Then the parasite settles into its dormant cysts and, for most people, that is the end of the story. Antibodies stay in the blood for life, which is how surveys can tell who has been infected.

The three situations that matter

A first infection during pregnancy. If a woman catches the parasite for the first time while pregnant, it can cross the placenta. The consequences for the child range from nothing to eye and brain damage¹¹¹³. Women infected before pregnancy are, with rare exceptions, protected. The pregnancy page covers the real risks and the precautions that work.

A weakened immune system. The dormant cysts are held in check by the immune system, day after day, for life. If that control fails, in advanced HIV, after an organ transplant, or during certain cancer treatments, the parasite can wake up and cause severe disease, most often in the brain¹¹.

The eye. Toxoplasma can infect the retina and leave scars that affect vision. This can happen to otherwise healthy people, and it is the most common infection of the retina in the United States¹⁴. It is far more frequent in parts of South America¹⁵¹⁶. See the eye page.

There is a fourth situation that most sources leave out. In parts of South America the parasite strains are different from those in Europe and North America, and some of them can cause serious illness even in healthy adults¹⁷. This is the subject my lab has worked on for twenty years, and it has its own page.

What you can do

For most people, nothing special is needed. If you are pregnant or planning to be, or your immune system is weakened, a short list of precautions cuts the risk substantially: cook meat properly or freeze it first, wash fruit and vegetables, wash hands after gardening, avoid untreated water and raw shellfish, and change the litter box daily so that oocysts never get the day or more they need to become infectious¹⁸. The food and water page explains why each of these works.

Sources

1. Montoya JG, Liesenfeld O. Toxoplasmosis. Lancet 2004;363:1965-76. PubMed · DOI
2. Jones JL, Kruszon-Moran D, Elder S et al. Toxoplasma gondii Infection in the United States, 2011-2014. Am J Trop Med Hyg 2018;98:551-557. PubMed · DOI
3. Centers for Disease Control and Prevention. About Toxoplasmosis. Accessed 17 September 2026.
4. Pappas G, Roussos N, Falagas ME. Toxoplasmosis snapshots: global status of Toxoplasma gondii seroprevalence and implications for pregnancy and congenital toxoplasmosis. Int J Parasitol 2009;39:1385-94. PubMed · DOI
5. Dubey JP. Duration of immunity to shedding of Toxoplasma gondii oocysts by cats. J Parasitol 1995;81:410-5. PubMed
6. Arranz-Solís D, Warschkau D, Fabian BT et al. Late Embryogenesis Abundant Proteins Contribute to the Resistance of Toxoplasma gondii Oocysts against Environmental Stresses. mBio 2023;14:e0286822. PubMed · DOI
7. Cook AJ, Gilbert RE, Buffolano W et al. Sources of toxoplasma infection in pregnant women: European multicentre case-control study. European Research Network on Congenital Toxoplasmosis. BMJ 2000;321:142-7. PubMed · DOI
8. Jones JL, Dargelas V, Roberts J et al. Risk factors for Toxoplasma gondii infection in the United States. Clin Infect Dis 2009;49:878-84. PubMed · DOI
9. Bowie WR, King AS, Werker DH et al. Outbreak of toxoplasmosis associated with municipal drinking water. The BC Toxoplasma Investigation Team. Lancet 1997;350:173-7. PubMed · DOI
10. Boyer K, Hill D, Mui E et al. Unrecognized ingestion of Toxoplasma gondii oocysts leads to congenital toxoplasmosis and causes epidemics in North America. Clin Infect Dis 2011;53:1081-9. PubMed · DOI
11. Centers for Disease Control and Prevention. People at Increased Risk for Toxoplasmosis. Accessed 17 September 2026.
12. Elmore SA, Jones JL, Conrad PA et al. Toxoplasma gondii: epidemiology, feline clinical aspects, and prevention. Trends Parasitol 2010;26:190-6. PubMed · DOI
13. Montoya JG, Remington JS. Management of Toxoplasma gondii infection during pregnancy. Clin Infect Dis 2008;47:554-66. PubMed · DOI
14. Jones JL, Holland GN. Annual burden of ocular toxoplasmosis in the US. Am J Trop Med Hyg 2010;82:464-5. PubMed · DOI
15. Holland GN. Ocular toxoplasmosis: a global reassessment. Part I: epidemiology and course of disease. Am J Ophthalmol 2003;136:973-88. PubMed · DOI
16. Glasner PD, Silveira C, Kruszon-Moran D et al. An unusually high prevalence of ocular toxoplasmosis in southern Brazil. Am J Ophthalmol 1992;114:136-44. PubMed · DOI
17. Carme B, Bissuel F, Ajzenberg D et al. Severe acquired toxoplasmosis in immunocompetent adult patients in French Guiana. J Clin Microbiol 2002;40:4037-44. PubMed · DOI
18. Centers for Disease Control and Prevention. Preventing Toxoplasmosis. Accessed 17 September 2026.