The bacterium, Vibrio parahaemolyticus, causes diarrhea, usually within 24 hours of eating infected seafood, along with abdominal cramping, nausea, and headache in most people, according to Dr. Nicholas A. Daniels and associates at the University of California, San Francisco.

"Shellfish, particularly oysters since they are filter-feeders, concentrate the bacteria in their tissues," Dr. Daniels told Reuters Health. "Humans become infected when they eat raw or undercooked shellfish. Each year in the United States, Vibrio cause an estimated 8,000 infections and 50 deaths."

• The authors investigated more than 400 cases of Vibrio-associated diarrhea that occurred between May 31 and July 10, 1998.

• The strain of Vibrio bacteria grown from 28 people was type O3:K6, which has never been seen before in US shellfish.

• Although oysters from the harvest sites were found to contain well below the allowable limit of total bacteria counts (10,000 per gram of oyster meat), half of the oysters contained low levels of the dangerous Vibrio bacteria.

The researchers also theorize that the rising seawater temperatures and salinity levels may have contributed to this large outbreak, since both of these had gone up during the year of the outbreak.

The researchers advise consumers to "thoroughly cook oysters and avoid eating raw or undercooked oysters, particularly in the warmer months."

The Journal of the American Medical Association 2000; 284: 1541-1545.