Elizabeth Weise of USA Today reports today that a single strain of Salmonella Typhimurium has sickened 336 people in 34 states, sending some to the hospital, over the past three months, but no one knows the source.

The outbreak led the Centers for Disease Control and Prevention to activate its emergency network this week, pulling in staff from other areas to work on the situation, says Frederick Angulo, deputy chief of enteric diseases.

"The lead hypothesis is chicken, but it’s hard thing to prove. Everybody eats chicken," Angulo says.