This year is shaping up to be a bad one for the spread of cyclospora, a parasite transmitted through contaminated food and water that causes severe diarrhea.

At least 1,645 people across 34 states have been infected, the Centers for Disease Control and Prevention reported on Tuesday. And 141 people have been hospitalized.

C.D.C. officials said that they are also aware of more than 5,100 additional cases nationwide that they have not yet confirmed — and these numbers are likely to grow.

“We anticipate continuing to see cases increase, possibly through the end of August,” said Gwen Biggerstaff, the deputy director of the C.D.C.’s Division of Foodborne, Waterborne, and Environmental Diseases in a news conference on Tuesday.

Michigan is currently reporting the most cyclospora infections in the country: 3,309 cases and 44 hospitalizations as of Tuesday morning, the highest number of cases ever reported in Michigan in a single year. The state typically records only 40 to 50 cases per year.

This year is “on track to be the biggest year on record” for cyclosporiasis in the United States, said Caitlin Rivers, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health who is tracking the cases. Since 2016, the C.D.C. has reported an average of about 2,800 cases of cyclosporiasis per year nationwide.

At least 400 cases in four states — Michigan, Ohio, West Virginia and Kentucky — appear to be linked to a common but unknown source, C.D.C. officials said on Tuesday.

On Monday, health officials in Michigan identified lettuce or salad greens as a potential source of the outbreak in that state, though they emphasized that the source was not definitive and others may be identified. Federal officials said Tuesday that they were investigating multiple types of produce — including lettuce — but have not yet found a specific source.

Some Taco Bell locations have removed certain ingredients — including lettuce, cilantro, onion, pico de gallo and guacamole — from their menus. In an emailed statement, the chain said that was “a precautionary measure” and that health officials have not confirmed a link between Taco Bell and the illnesses.

Cyclospora infections rarely cause serious complications and are not typically life-threatening, said Dr. Thomas Moore, an infectious disease specialist and clinical professor of medicine at the University of Kansas School of Medicine-Wichita. However, some people — especially those in vulnerable groups — may require hospitalization, such as for dehydration. It’s a good reminder to take food safety seriously, Dr. Moore said.

How can you reduce your risk?

Past cyclospora outbreaks in the United States have mainly been linked to fresh produce, including raspberries, basil, cilantro, fruit mixes, snow peas, snap peas, lettuce and salad mixes. They have been caused by both imported and domestic fruits and vegetables and those prepared at home and in restaurants, said Donald Schaffner, chair of the food science department at Rutgers University.

Because the cause of the recent infections is uncertain, the best way to prevent illness is to follow standard hygiene and food safety practices, said Dr. Adam Ressler, an infectious disease physician with Michigan Medicine.

Always wash your hands with soap and water after using the bathroom and before preparing food, Dr. Moore said. Alcohol-based hand sanitizers do not kill cyclospora, he said, but soap and water does.

Rinsing fruits and vegetables with water is always a good practice, Dr. Moore said. Some cyclospora may still cling to contaminated produce after rinsing, but it will reduce their numbers and in turn can reduce your risk of getting sick or make the illness less severe, he said.

Cleaning produce with vinegar or other solutions is generally no more effective than doing so with running water, Dr. Schaffner said.

Produce is safest when heated to 158 degrees or higher, which kills cyclospora, according to Michigan’s health department.

Given the early signs that lettuce may be implicated in the Michigan outbreak, health officials there have recommended using whole heads of lettuce rather than bagged lettuce or pre-mixed salad kits. Discard the outer two to three layers of leaves and then wash the inner leaves thoroughly under running water.

The department has also issued advice for other types of produce that have been implicated in previous outbreaks:

• Cilantro and basil should be washed thoroughly under running water, while separating the leaves.

• For green onions, trim the root end and remove the outer layer, and wash thoroughly under running water.

• Snow peas should be washed under running water while rubbing the surface.

• Raspberries are difficult to clean because their bumpy surface provides crevices where the parasite can hide. Consider cooking them in pies or jam. Freezing berries may reduce the number of parasites, though probably not entirely.

You can’t eliminate your risk completely, Dr. Schneider said. “There is an inherent risk with any raw food that you eat.”