WASHINGTON – Donna Heller thought she had cancer. But multiple visits to the doctor after a month with debilitating nausea and diarrhea didn’t yield any answers. Convinced she was dying, she met with her lawyer to get her will in order.

Then she saw a television report about an outbreak of cyclospora possibly linked to bagged salad mix. The stomach illness matched all her symptoms and is easily treatable with antibiotics. She told her doctor she suspected that could be the cause, and tests showed she was right.

“It went so long and nobody was able to give me answers,” said Heller, a 54-year-old teacher in Crowley, Texas. “It didn’t seem like anybody wanted to take you serious because there are so many stomach problems that resemble each other.”

A mysterious outbreak of the parasitic illness usually found abroad is growing, with more than 400 confirmed cases in 16 states. FDA officials said Friday they had discovered the source of some of the illnesses, but not all of them.

The FDA said it traced illnesses from restaurants in Nebraska and Iowa to Taylor Farms de Mexico, the Mexican branch of Salinas, Calif.-based Taylor Farms. The company, which provides produce to the food service industry, said its facility located about 180 miles north of Mexico City in San Miguel de Allende is the only one of its 12 sites to be connected to the cases.

In an email, the chairman and CEO of Taylor Farms, Bruce Taylor, said the Mexican plant produced 48 million servings of salads for thousands of restaurants in the Midwest and eastern United States in June, the month the outbreak started. He said the facility has an extensive water testing program.

The rest of the illnesses — many of them in Texas — are still a mystery, state and federal officials say.

The source of this outbreak has proved particularly hard to trace. Doctors have to test specifically for cyclospora and many don’t because it is relatively rare. So they may not order the correct tests, at least not at first.

The parasite is so tiny that it’s often difficult to confirm that a person has the illness, according to the Centers for Disease Control and Prevention. Tests often have to be repeated with fresh samples.

Heller said initial tests from her doctor showed up as inconclusive, but she later received a call from the CDC telling her she definitely had the illness.

Doctors or labs may not notify state health departments as quickly as they would for a more common foodborne illness such as salmonella. And there are different rules in different states about whether cyclospora has to be reported to federal health authorities.

All those obstacles are making it harder for state and federal officials. It also means there are probably many people who have the disease and don’t know it.

The illness is rare – roughly 150 cases are reported in the United States annually. Scientists only identified it in the early 1990s.

The cyclospora parasite is native to the tropics and tends to come into the United States on imported produce.

For example, Guatemalan raspberries were the source of five outbreaks in Canada and the United States in the late 1990s. Two of those outbreaks involved more than 1,000 illnesses each, said Ynes Ortega, a cyclospora expert at the University of Georgia.

Heller says she doesn’t know what food might have caused her illness, but she said she was eating out a lot near her home 13 miles south of Fort Worth around the time she fell ill in late June.

She said she finally went on the correct antibiotics last week and is starting to feel better, although her symptoms aren’t gone completely. She said the illness has taken an emotional toll.

“I literally, through the course of all this, have been brought to tears probably 10 different times, just so defeated,” she said.