Toxoplasmosis is one of the most common parasitic infections worldwide. Roughly one in three people globally have been exposed to the parasite that causes it, based on antibody testing in pregnant women across dozens of countries. In the United States, seroprevalence sits between 20% and 40% of the adult population, meaning tens of millions of Americans carry the parasite. The vast majority never know it.
Global Infection Rates
A large meta-analysis of studies from around the world found that about 33% of pregnant women tested positive for past infection with the Toxoplasma gondii parasite. That figure serves as a useful proxy for the general adult population, since pregnant women are routinely screened in many countries. The rate of active or recent infection was much lower, around 1.9% globally, reflecting how many people pick up the parasite in any given period.
Rates vary dramatically by region. Countries with warm, humid climates and traditions of eating raw or undercooked meat tend to have higher seroprevalence. France, for example, has historically had some of the highest rates in Europe. Meanwhile, the United States has one of the lowest rates of active new infection among industrialized nations, estimated at just 0.01% of pregnant women showing signs of recent exposure.
Most People Never Have Symptoms
About 90% of infections in people with healthy immune systems produce no symptoms at all, or cause such mild, nonspecific illness that they’re never diagnosed. When symptoms do appear, they typically resemble a mild flu: swollen lymph nodes, low-grade fever, muscle aches, and fatigue that resolve on their own within a few weeks. The parasite then forms dormant cysts in muscle and brain tissue, where it persists for life without causing problems in most people.
This is why toxoplasmosis is considered heavily underdiagnosed. The CDC has noted that the infection’s tendency to be asymptomatic or self-limiting makes routine surveillance difficult. Many people only discover they’ve been infected when a blood test reveals antibodies, often during pregnancy screening or before an organ transplant.
Who Faces Serious Risk
Toxoplasmosis becomes dangerous in two main situations: when someone’s immune system is severely compromised, or when a pregnant person catches the parasite for the first time during pregnancy.
For people with advanced HIV who carry the dormant parasite, the consequences can be severe. Before effective antiretroviral therapy became available, roughly one in three HIV patients with both advanced immune suppression and existing Toxoplasma infection developed a brain infection called toxoplasmic encephalitis within a year. Modern HIV treatment has dramatically reduced this risk, but the parasite remains a concern for anyone whose immune system is significantly weakened, including organ transplant recipients on immunosuppressive drugs.
Risk During Pregnancy
When a person contracts Toxoplasma for the first time while pregnant, the parasite can cross the placenta and infect the developing fetus. In most industrialized countries, congenital toxoplasmosis occurs in 1 to 10 out of every 10,000 live births. In the United States, this translates to an estimated 400 to 4,000 cases per year. France, with its higher overall infection rate, sees roughly 600 congenital cases annually.
The effects on the baby depend heavily on timing. Infections earlier in pregnancy are less likely to reach the fetus but tend to cause more severe damage when they do, potentially affecting the brain and eyes. Infections later in pregnancy cross the placenta more easily but often cause milder or no symptoms at birth, though problems can emerge months or years later.
Eye Complications
One of the more common long-term effects of toxoplasmosis is ocular disease, where the parasite damages the retina. Among people infected with Toxoplasma, roughly 2% develop some form of eye involvement. During one well-documented outbreak in British Columbia, symptomatic retinal inflammation appeared in 0.2% to 0.7% of those infected.
These rates may sound small, but given how many people carry the parasite globally, ocular toxoplasmosis is actually one of the leading infectious causes of vision impairment. Symptoms include blurred vision, floaters, eye pain, and sensitivity to light. Episodes can recur, and each flare-up risks additional scarring of the retina.
How People Get Infected
Toxoplasma gondii has two main routes into humans: eating undercooked or raw meat containing parasite cysts, and accidentally swallowing the microscopic eggs shed in cat feces (through contaminated soil, water, or unwashed produce). The exact proportion of infections caused by each route isn’t known for the general population, a gap researchers have acknowledged. Both pathways are well-established contributors.
Cats are the only animals that shed the infectious form of the parasite in their feces, but they typically only do so for a few weeks after their initial infection. The bigger everyday risk for most people is likely dietary. Pork, lamb, and venison are among the meats most commonly found to harbor Toxoplasma cysts, though proper cooking to an internal temperature of 165°F kills the parasite. Freezing meat for several days before cooking also reduces risk.
Gardening in contaminated soil, drinking untreated water, and eating unwashed fruits or vegetables are additional exposure routes, particularly in areas where outdoor cats are common.
Toxoplasmosis as a Foodborne Threat
Despite flying under the radar for most people who carry it, toxoplasmosis is a meaningful public health concern. The CDC estimates it causes approximately 44 deaths per year in the United States from foodborne transmission alone. While that number is lower than the top foodborne killers like Salmonella and Listeria, it places Toxoplasma among the more consequential food-related parasites in the country. The overall disease burden is considered significant because of the severity of outcomes in vulnerable populations, particularly the potential for lifelong disability in congenitally infected children.