WASHINGTON — It started with a few sick Floridians in May. Then international business executives meeting in Texas all returned home with diarrhea.

Now the parasite called cyclospora has hit an estimated 1,000 people as government disease detectives struggle to find the source.

The outbreak is an example of how vulnerable the United States is to new diseases, scientists say — especially ones that catch a ride on the food supply.

“We did get lucky” with cyclospora because a few alert doctors sounded the alarm early, said Dr. Stephen Ostroff of the Centers for Disease Control and Prevention.

But cyclospora is a reminder that “the potential for a massive outbreak is certainly there,” he said.

Cyclospora is a single-celled parasite that invades the small intestine and causes diarrhea, vomiting, weight loss, fatigue and muscle aches. Although treatable with antibiotics, it can last several weeks and cause dehydration while symptoms last. Some patients have been hospitalized.

It first made headlines two weeks ago when Texas health officials blamed a new outbreak of diarrhea on California strawberries. But the CDC then backed off strawberries, saying laboratory tests hadn’t found any that were contaminated.

By this week, reports of illness had reached 1,000 people in the United States and Canada. Fresh fruit, including raspberries and other berries, is a suspect because clusters of patients who were sickened at weddings, catered luncheons and even at restaurants all say they ate fruit mixes.

The good news: Washing all fresh produce — a must to fight any germs — will clean away cyclospora. If it really lurks on fruit, cooking would make jams or jellies safe.

And while the CDC still is getting new reports of patients, most are well by now — it just took their doctors a while to suspect cyclospora and report them. That means the outbreak probably is waning, Ostroff said.

Still, scientists at the CDC and the Food and Drug Administration are working around the clock this holiday week at laboratories in five states. They’re using the “DNA fingerprinting” process to find any trace of cyclospora on hundreds of samples of produce, or the soil and water used to grow it.

To date, they haven’t found cyclospora on a single piece of food, nor in soil or water, said FDA Deputy Commissioner Mary Pendergast.

The tracking is hard because it takes longer to get sick from cyclospora — about a week — than from many other food-borne pathogens. Testing each food sample can take over 14 hours, Pendergast said.

The CDC had hoped to avoid such outbreaks when, two years ago, it started the nation’s first early-warning system for emerging diseases.

The idea is for mysterious illnesses to trigger state health officials to ask the right questions and do the right tests to quickly spot a new infection — like the rodent-borne hantavirus that struck the Southwest in 1992, or the cryptosporidium that sickened 400,000 Milwaukee residents the following year.

But that system is still in its infancy, and some states are discussing turning their public health laboratories over to private companies to save money.

The CDC successfully lobbied against that move when Pennsylvania debated it earlier this year. But others still are considering it, Ostroff said. Without those laboratories, the CDC wouldn’t have enough help in testing for new health threats, he said. That is especially true for food-borne threats that, because of their wide distribution, can sicken lots of people quickly.

Cyclospora, for example, “wasn’t on anybody’s radar screen,” he said.

It had never been seen in humans until 1977, and then it wasn’t thought to cause disease. Only in the late 1980s did doctors in the tropics notice it causing travelers’ diarrhea.

In this country, about 20 people in Chicago were sickened by cyclospora from contaminated drinking water in 1990; two other small outbreaks were reported in New York and Florida last year.

So when Floridians got sick in May, their doctors knew to test for cyclospora. A Canadian doctor called to report patients just back from Texas who must have picked up cyclospora there. Then federal scientists knew to start tracking a suspected source.