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.

Donna Heller from Burleson, Tex., suffered debilitating nausea and diarrhea lasting a month until she was diagnosed with cyclospora by the CDC last week and began taking the antibiotics. (LM Otero/Associated Press)

"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 agency said that the illnesses from Iowa and Nebraska are linked to salad mix from a Mexican farm that was served at Olive Garden and Red Lobster restaurants. Those make up around half of the cases.

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.

Many have the disease and don't know it

Doctors or labs may not notify state health departments as quickly as they would for a more common foodborne illness like 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.

There are usually only 150 cases a year in the U.S. of cyclospora poisoning. (Centers for Disease Control/Associated Press)

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

In comparison, there are tens of thousands of lab-confirmed cases of salmonella food poisoning in this country each year, and officials believe there are hundreds of thousands more that are not confirmed.

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.

Takes a week to show up

Officials say part of the problem is that the disease takes a week on average to show up, and diagnosis has often been delayed, making it hard for victims to remember what they ate.

CDC spokeswoman MarDia-Belen Moran says the agency also is interviewing people who aren't sick as controls to get more information on eating patterns, as well as lab testing foods that they suspect.

For its part, the FDA says it has a 21-person team in its Maryland headquarters and specialists in 10 field offices across the country working to identify the source of the outbreak.

Food often goes through several stops — potentially in several countries — before it reaches a grocery cart, and trying to trace it is "labour-intensive and painstaking work, requiring the collection, review and analysis of hundreds and at times thousands of invoices and shipping documents," the FDA said.

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 south of Fort Worth around the time she fell ill in late June.

She said she finally went on the correct antibiotics this week and is starting to feel better, though 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.