DES MOINES, Iowa – The outbreak of a stomach bug two states have linked to bagged salad came as little surprise to food safety experts, who say the process of harvesting, washing and packaging leafy greens provides numerous opportunities for contamination.

Although nutritionists stress the chances of getting sick from vegetables are low compared to the dangers of a diet without them, packaged salads heighten the risk because leaves from several batches often are mixed together.

“The washing and comingling of different batches of lettuce means a hazard that may appear in one field can show up in lots of bags of lettuce because of the common bath,” said Caroline Smith DeWaal, director of the food safety program for the Center for Science in the Public Interest, a consumer health advocacy organization based in Washington.

Officials in Iowa and Nebraska say a packaged salad mix containing iceberg and romaine lettuce, carrots and red cabbage was infected with cyclospora, a parasite blamed for sickening 397 people in 16 states. It’s not clear whether the produce also was to blame for the outbreak in the other states.

Last year the Food and Drug Administration issued more than 20 recalls for packaged salads, romaine lettuce or spinach. Most were due to tests finding listeria or E. coli bacteria, both of which can cause serious illness.

However, of the 693 food product recalls between October 2011 and September 2012 — the last available year of records — only about 15 pertained to bagged lettuce or salads.

The U.S. Centers for Disease Control and Prevention has estimated that one in six Americans — 48 million people — get sick from foodborne illnesses each year. About 128,000 are hospitalized and 3,000 die.

In March, the CDC released a study that looked at more than 4,500 food-related outbreaks between 1998 and 2008 and found more illnesses attributed to leafy vegetables — 22 percent — than to any other food. The agency didn’t say what percentage of those was packaged.

Dr. Robert Tauxe, the CDC’s deputy director of foodborne, waterborne and environmental diseases, said the industry that cuts and bags fresh produce has made significant improvements in its processes since 2006. An outbreak that year tied to E. coli-contaminated spinach caused three deaths and sickened 205 people. “A lot has been done so that actually the bagged lettuce-type produce is a good deal safer now that it was five years ago,” he said.

Michael Doyle, director of the University of Georgia’s Center for Food Safety, said lettuce is susceptible to contamination because it grows close to the ground and is more susceptible to microbial contamination. Water used for irrigation can be contaminated, and there could be issues with workers lacking good hygienic practices.