Toxoplasmosis is an infection caused by a single-celled parasite that can invade virtually any cell in your body, form dormant cysts in your brain and muscles, and stay there for life. About 11 percent of the U.S. population age 6 and older has been infected. Most people never realize it, but for pregnant women, people with weakened immune systems, and in some cases even healthy adults, the parasite can cause serious damage to the brain, eyes, and developing fetus.
How the Parasite Gets Inside Your Cells
Toxoplasma gondii is an obligate intracellular parasite, meaning it can only survive and reproduce inside living cells. What makes it unusually dangerous is its lack of pickiness: it can invade any nucleated cell in any warm-blooded animal, including every tissue type in the human body.
Once the parasite contacts a human cell, it secretes specialized proteins that form a temporary junction between itself and the cell membrane. It then pushes through this junction and creates a protective bubble inside the cell called a parasitophorous vacuole. This bubble is the key to the parasite’s survival. It prevents the cell’s normal defense systems from destroying the invader, giving the parasite a safe space to multiply. Eventually the cell ruptures, releasing new parasites that spread to neighboring cells.
After your immune system mounts a response, the parasite shifts strategy. Instead of actively replicating, it forms slow-growing tissue cysts, primarily in the brain, skeletal muscles, and heart. These cysts can persist for decades, possibly for the rest of your life, in a dormant state that largely evades immune detection.
What It Does in Healthy Adults
Most people with a functioning immune system never notice the infection at all. When symptoms do appear, they typically resemble a mild flu: swollen lymph nodes (especially in the neck), muscle aches, fatigue, and low-grade fever. These symptoms usually resolve on their own within a few weeks without treatment. After that, the parasite settles into its dormant cyst phase, and the immune system keeps it in check indefinitely.
This doesn’t mean the parasite is doing nothing. Even in its dormant state, Toxoplasma has been shown to alter neurotransmitter activity in the brain, particularly dopamine. The parasite increases dopamine production by boosting the activity of a key enzyme involved in dopamine synthesis. This has led researchers to investigate whether chronic, latent infection contributes to subtle changes in behavior and mental health, a topic covered in more detail below.
Serious Risks for Immunocompromised People
When the immune system is suppressed, whether from HIV/AIDS, organ transplant medications, or chemotherapy, those dormant cysts can reactivate. The most dangerous result is toxoplasmic encephalitis, an inflammation of the brain that can develop rapidly and become life-threatening.
Symptoms of reactivated brain infection include severe headache, confusion, muscle weakness, fever, and seizures. In advanced cases, it can progress to coma. Before effective preventive treatments became standard for HIV patients, toxoplasmic encephalitis was one of the most common opportunistic brain infections in people with AIDS. It remains a serious concern for anyone whose immune system is significantly weakened.
Damage to the Eyes
Toxoplasmosis is one of the most common infectious causes of retinal inflammation worldwide. The parasite targets the retina, creating an active inflammatory lesion that eventually heals into a scar. Each episode of inflammation damages more retinal tissue, and if the scarring reaches the central part of the retina responsible for sharp vision, it can lead to progressive and potentially permanent vision loss.
Symptoms of ocular toxoplasmosis include blurred or reduced vision, eye pain (especially in bright light), redness, and tearing. The eye disease can reactivate months or years after the initial infection, with each flare-up causing additional damage. Among people infected before birth (congenitally), 20 to 80 percent develop retinal lesions by adulthood. In people infected after birth, the rate is much lower, under 2 percent in the United States. Treatment for active eye disease typically involves a combination of antiparasitic medications given for four to six weeks.
What It Does to a Developing Fetus
Congenital toxoplasmosis, when the parasite passes from a newly infected mother to her unborn baby, is where the infection causes its most devastating harm. The severity depends heavily on timing. Infection during the first trimester tends to produce the most serious damage, though transmission is actually less likely early in pregnancy. Later in pregnancy, transmission becomes more likely but the effects are generally milder.
The classic pattern of severe congenital toxoplasmosis involves three hallmark problems: inflammation and scarring of the retina (chorioretinitis), fluid buildup in the brain (hydrocephalus), and calcium deposits in brain tissue (cerebral calcifications). In reality, this full triad appears in only a limited number of infected newborns. The broader range of possible effects includes:
- Neurological problems: seizures, abnormal head size, brain inflammation
- Eye damage: retinal scarring, crossed eyes, abnormally small eyes
- Systemic signs: enlarged liver and spleen, jaundice, rash, anemia, low platelet counts
Some severely affected babies die in utero or within days of birth. But many congenitally infected infants appear healthy at birth, only to develop problems later in childhood or adolescence. These delayed effects can include recurring retinal inflammation leading to vision loss, learning disabilities, intellectual disability, hearing loss, motor delays, and hormonal disruptions from damage to the hypothalamus and pituitary gland. This is why screening and early treatment in pregnancy matter so much, even when the newborn initially looks fine.
Links to Mental Health and Behavior
The parasite’s ability to form cysts in the brain and manipulate dopamine has drawn significant scientific attention. Meta-analyses have found associations between latent Toxoplasma infection and several psychiatric conditions, including schizophrenia, obsessive-compulsive disorder, addiction, anxiety disorders, PTSD, and certain personality disorders (particularly antisocial and borderline types). In schizophrenia patients specifically, latent infection has been linked to more severe psychotic symptoms.
Research published in Nature found that Toxoplasma exposure in people with severe mental illness was associated with elevated cortisol levels, suggesting the parasite may also influence the body’s stress response system. However, the relationship is complex and context-dependent. In schizophrenia patients with long-standing illness, latent infection was paradoxically associated with lower symptom scores, possibly because of long-term antipsychotic use or some adaptation in how the parasite affects brain function over time. The bottom line: there is real evidence that the parasite influences brain chemistry, but the clinical significance for any individual person remains an active area of investigation.
How People Get Infected
Cats are the only animals in which Toxoplasma completes its reproductive cycle, shedding infectious eggs (oocysts) in their feces. But direct cat contact is not the most common route of human infection. You’re more likely to get toxoplasmosis from eating undercooked or raw meat containing tissue cysts, or from accidentally ingesting oocysts through contaminated soil, water, or unwashed produce. Organ transplants and blood transfusions are rare but possible transmission routes.
Cooking meat to proper temperatures destroys the parasite. The USDA recommends cooking whole cuts of meat (excluding poultry) to at least 145°F (63°C) with a three-minute rest before cutting. Ground meat should reach 160°F (71°C), and all poultry should hit 165°F (74°C). Washing fruits and vegetables, wearing gloves while gardening, and having someone else clean the litter box during pregnancy are all effective prevention strategies.
How Toxoplasmosis Is Treated
Healthy adults with mild or no symptoms typically don’t need treatment. The immune system controls the infection on its own, and the dormant cysts that remain are not actively causing harm in most people.
When treatment is needed, for active eye disease, severe symptoms, congenital infection, or reactivation in immunocompromised patients, the standard approach combines an antiparasitic drug with an antibiotic. Because the primary medication can suppress bone marrow function, patients also take a protective supplement (folinic acid) alongside it to prevent that side effect. A typical course of treatment lasts four to six weeks, after which the situation is reassessed. For immunocompromised patients, ongoing maintenance therapy may be necessary to prevent the infection from flaring up again.