Health officials think the bacteria may have spread through some form of produce — the list of suspects includes tomatoes. But the illnesses have not been tied to any specific product, chain, restaurants or supermarkets.
No one has died in the outbreak, which stems from a common form of salmonella bacteria. Eleven people have been hospitalized, health officials said.
"We're very early in the investigation," said Dave Daigle, a spokesman for the U.S. Centers for Disease Control and Prevention.
The CDC first detected the outbreak two weeks ago through a national computer lab system that looks for patterns and matches in reports of food-borne illness. The Food and Drug Administration has joined the investigation and will try to help trace the outbreak to its origin.
Most of the cases are in adults, and more than 60 percent are women, said Dr. Chris Braden, a CDC epidemiologist investigating the outbreak.
The states involved are Arkansas, Connecticut, Georgia, Indiana, Kentucky, Maine, Massachusetts, Michigan, Minnesota, North Carolina, New Hampshire, Ohio, Pennsylvania, Rhode Island, Tennessee, Virginia, Vermont and Wisconsin.
Salmonella generally causes a nonfatal, diarrhea-causing illness. Other symptoms can include nausea, vomiting, abdominal cramps, fever and headache.
There are about 2,500 types of salmonella. The type in this outbreak — Salmonella typhimurium — is one of the most common, Braden said.
People can catch the infection from many different sources, including water, soil, insects, factory surfaces, kitchen surfaces, animal feces, and raw meats, poultry and seafood.
Outbreaks of food-borne illness have repeatedly made headlines this year. Certain brands of packaged spinach, lettuce, carrot juice, beef and unpasteurized milk recently were recalled after they were found to be tainted with illness-causing bacteria.
The most serious outbreak, first reported in September, involved spinach tainted with E. coli bacteria that killed three people and sickened more than 200.
Health officials estimate that more than 1.4 million cases of salmonellosis occur in the United States each year. About 1.3 million of those cases come from food, Braden said.