A salmonella outbreak that has sickened hundreds of people across 37 states is being caused by whole onions, the Centers for Disease Control and Prevention said.The CDC said the whole red, white and yellow onions were imported from Chihuahua, Mexico, and sold to restaurants and grocery stores throughout the United States.The onions were last imported on Aug. 27, but the company behind the distribution said they can last up to three months in storage and may still be in homes and businesses.The onions may have stickers or packaging indicating the brand (ProSource Inc.) and the country (Mexico) where they were grown.Consumers are urged to check storage coolers for the onions."If you can’t tell where they are from, throw them away," the CDC said. Just over 650 people across the country have gotten sick due to the contaminated vegetables. Most people infected with salmonella experience diarrhea, fever and stomach cramps, according to the CDC. Symptoms usually start six hours to six days after swallowing the bacteria.Investigators are working to determine if other onions and suppliers are linked to this outbreak.