Federal health officials said a single strain of salmonella has sickened 388 people in 42 states since the fall, but the cause of the outbreak remains unknown.

The outbreak started Sept. 3, but most patients have become ill since mid-October, Lola Russell, a spokeswoman for the federal Centers for Disease Control and Prevention, said Wednesday. Ms. Russell didn't give a number for how many were hospitalized, but said it was 18% of those interviewed.

The...