SACRAMENTO, Calif. (AP) — The U.S. Department of Agriculture on Monday issued a public health alert for raw chicken packaged at three Foster Farms facilities in California after 278 people fell ill. Strains of Salmonella Heidelberg are associated with chicken distributed to retail outlets in California, Oregon and Washington state, the USDA said in a statement. The Salmonella outbreak has spread to 18 states, though most of the reported illnesses have been in California. The outbreak appears to have begun in March and the USDA was notified of the illnesses in July, said Dan Engeljohn of the USDA's Food Safety and Inspection Service. Investigators had a difficult time pinpointing the source of the illnesses, Englejohn said. A spokesman for Foster Farms says no recall was in effect and that the infections were caused by eating chicken that was undercooked or improperly handled. The USDA has not directly linked the outbreak of illnesses to a specific product or production period. The USDA mark on suspect packages would read: P6137, P6137A and P7632. Messages left with California state health officials were not immediately returned. Cooking chicken to 165-degrees kills the Salmonella bacteria.