Q. How worried should cat owners be about the parasite Toxoplasma gondii, especially with babies in the house.

A. The only people who face a risk from Toxoplasma gondii are pregnant women who have not previously been infected, babies under 6 months old and any household member whose immune system has been weakened by cancer treatment, transplant therapy or an infection like H.I.V.

About 20 percent of the American public is infected with Toxoplasma gondii, a parasite that can infect birds and most other animals but that reproduces sexually only in cats. The parasite typically remains dormant in people after an initial few days of mild flu-like symptoms, said Dr. Michael Grigg, a senior investigator with the National Institute of Allergy and Infectious Diseases. If the dormant parasite becomes active, causing the disease known as toxoplasmosis, it can result in neurological problems, such as seizures.

“It is quite possibly the most successful parasite on the planet,” Dr. Grigg said, but if you have a working immune system, “you really have almost nothing to worry about.”

A previously infected woman who gets pregnant will not have a problem, because her immune system will keep the infection in check, said Dr. Rima McLeod, director of the toxoplasmosis center at the University of Chicago. She will also pass that immunity on to her unborn child.

But a first infection during pregnancy will cross the placenta, Dr. Grigg said, potentially leading to fetal death, stillbirth or problems in a newborn, including an enlarged head, cognitive deficits and almost certainly eye disease. Newborns born to mothers without previous infection are also vulnerable to the parasite, he said.

“This can be a very serious infection,” said Dr. McLeod, who is also a professor of ophthalmology and pediatrics at the University of Chicago. “It can cause a devastating disease in infants, with significant harm for them at birth and also later in life. It can have consequences for them and their families, lifelong.”

There are precautions a pregnant woman can take. If a test early in pregnancy shows she has not previously been infected, she should avoid changing cat litter herself and have someone else change it daily, using boiling water to disinfect the box, Dr. McLeod said.

“If they’re able to keep their cat inside while they’re pregnant and give the cat tin food that’s cooked, then they don’t need to worry,” she said. “The cat that’s a problem is the feral hunting cat or cat fed uncooked meat.”

An acutely infected cat or kitten can excrete in two weeks up to 500 million oocysts — the infectious form of the parasite — which can remain infectious in soil and water for up to a year, Dr. McLeod said. A person can get infected from even one of these oocysts. “It’s an amazingly effective dissemination system,” she said.

There are other ways to get infected besides direct contact with cats, including eating undercooked meat or unwashed fruits and vegetables, drinking unfiltered water, leaving sandboxes uncovered (where cats may defecate) or gardening without gloves.

“In all cases, I think pregnant women should be screened monthly by their obstetrician because there’s so much risk in the environment,” Dr. McLeod said.

Treatments can keep the parasite from doing damage, she said, but cannot get rid of it completely. Vaccines and curative treatments are under development, and Dr. McLeod hopes that someday Toxoplasma gondii will pose less of a threat to pregnant women, infants or those who are immunocompromised.