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- Posted January 29, 2026
Common Parasite Hiding in Many People Is More Complex Than Scientists Thought
A parasite that lives inside as many as 1 in 3 people worldwide may be much harder to treat than once believed, according to new research from the University of California, Riverside.
The study, published Jan. 24 in the journal Nature Communications, found that Toxoplasma gondii hides inside the body in far more complex ways than experts realized. It’s the parasite that causes the disease, toxoplasmosis.
The discovery may explain why current medicines cannot fully clear the infection.
People usually get toxoplasmosis by eating undercooked meat or touching contaminated soil or cat feces. Most infected people never feel sick.
But once inside the body, the parasite can stay for life.
It protects itself by forming tiny cysts, mostly in the brain and muscles. Each cyst can hold hundreds of dormant parasites.
These parasites can wake up later, especially in people with weak immune systems, sometimes leading to serious brain or eye problems. Infection during pregnancy can also harm the developing fetus.
Scientists long believed each cyst contained just one inactive type of parasite. Using advanced single-cell analysis, however, researchers discovered that every cyst actually holds several different parasite types, each with its own job.
"We found the cyst is not just a quiet hiding place — it's an active hub with different parasite types geared toward survival, spread or reactivation," study lead author Emma Wilson, a professor of biomedical sciences at UC Riverside, said in a news release.
Wilson said cysts slowly form as the immune system puts pressure on the parasite. Each cyst has a tough outer wall and is packed with slow-growing parasites called bradyzoites.
Cysts can grow up to 80 microns wide, and each bradyzoite measures about five microns long. (That’s minute: 80 microns are roughly equivalent to 0.003 of an inch.)
These cysts are most often found in neurons, cells that make up the nervous system. But they also appear in heart and skeletal muscle, which helps explain why people can become infected by eating undercooked meat.
Once cysts form, they resist all current treatments and stay in the body forever, Wilson said. They also help the parasite spread.
When cysts reactivate, the slow parasites turn into fast-multiplying forms called tachyzoites.
These can travel through the body and cause serious illness such as brain damage (toxoplasmic encephalitis) or vision loss (retinal toxoplasmosis).
Using a mouse model that closely mimicked natural infection, the research team isolated parasites directly from cysts. Each cyst had least five bradyzoite subtypes inside.
"For decades, the Toxoplasma life cycle was understood in overly simplistic terms, conceptualized as a linear transition between tachyzoite and bradyzoite stages," Wilson said. "Our research challenges that model. By applying single-cell RNA sequencing to parasites isolated directly from cysts in vivo, we found unexpected complexity within the cyst itself."
Current drugs can control the fast-growing parasite form but cannot kill cysts.
"By identifying different parasite subtypes inside cysts, our study pinpoints which ones are most likely to reactivate and cause damage," she said. "This helps explain why past drug development efforts have struggled and suggests new, more precise targets for future therapies."
Scientists have long linked toxoplasmosis to pregnancy complications. Congenital infection remains a serious risk when someone is first exposed during pregnancy.
Wilson hopes the findings will shift attention toward toxoplasmosis.
"Our work changes how we think about the Toxoplasma cyst," she said. "It reframes the cyst as the central control point of the parasite's life cycle. It shows us where to aim new treatments. If we want to really treat toxoplasmosis, the cyst is the place to focus."
More information
The Mayo Clinic has more on toxoplasmosis.
SOURCE: University of California, Riverside, news release, Jan. 27, 2026
