Chicken, turkey and other poultry accounted for 17 percent of the food-borne illness outbreaks reported to the government. Beef and leafy vegetables were close behind, at 16 percent and 14 percent.
The report from the Centers for Disease Control and Prevention covers outbreaks in 2007. Poultry was also the No. 1 source of outbreaks in 2006.
Salmonella and other kinds of bacteria caused about half of the outbreaks, the CDC said. Viruses -- like norovirus -- caused about 40 percent, mushroom toxin or other chemical agents were blamed for 7 percent. Parasites accounted for 1 percent.
Several things can cause an outbreak. For example, an infected person might contaminate the food while handling it. A contaminated food may be left out a room temperature for hours, allowing bacteria to multiply. It may not be cooked enough to kill the bacteria.
The CCD counted more than 21,000 illnesses in about 1,100 outbreaks in 48 states and Puerto Rico. There were 18 deaths from food poisoning.
It's far from a complete picture, however. Experts estimates that only about 5 percent of people who get food poisoning are part of a recognized outbreak, which is a cluster of two or more cases.
"It's just the tip of the iceberg," said Michael Doyle, head of the University of Georgia's food safety center.
An estimated 87 million cases of food-borne illness occur in the United States each year, including 371,000 hospitalizations and 5,700 deaths, according to an Associated Press calculation that combines a CDC formula with recent population estimates.
The CDC also closely tracks food-borne illnesses in 10 states to watch for food poisoning trends. Its report for last year showed rates of food-borne illness have been holding steady for the past five years.
In Thursday's report, the CDC only counted instances in which one food -- like grilled chicken -- was clearly to blame. That was the case in about 45 percent of the outbreaks. Outbreaks involving multiple ingredients -- like chicken salad -- were not part of that accounting.
A diarrhea-causing bacteria called Clostridium perfringens was commonly linked to poultry. A more deadly bacteria called E.coli O157:H7 was most often linked to beef.
Norovirus, a common bug most often spread by food handlers, was frequently seen in leafy vegetables.