A salmonella outbreak that swept 19 U.S. states in late 2006 has been a mystery, until now. Nearly 200 consumers were sickened by what investigators believed to be tainted tomatoes, or other produce.

But now, the Centers for Disease Control says the apparent source of the 2006 salmonella outbreak was tainted dog food. The CDC in recent months investigated a number of dry dog food brands and recalled two of them.

"This is the first time human illness has been linked to dry dog food," CDC epidemiologist Dr. Casey Barton Behravesh, told the Washington Post.

The 2006 outbreak hit Pennsylvania the hardest, with 29 reported cases. New York had nine cases while Ohio had seven. Nearly half those who got sick were very young children.

So, how could tainted dog food make people sick?

CDC investigators say the most likely scenario had pet owners feeding their dogs tainted, dry dog food before preparing a meal for humans. The CDC says animal owners should always wash their hands after touching pet food. The agency also advises keeping young children away from Fido's food bowl.

The Salmonella germ is actually a group of bacteria that can cause diarrhea illness in humans. They are microscopic living creatures that pass from the feces of people or animals, to other people or other animals.

The typical symptoms include diarrhea, fever and stomach pain, which start up to three days after people become infected.

The symptoms usually go away after one week. But some people have to see a doctor or be hospitalized because the diarrhea is severe or the infection has affected other organs.

The latest finding is likely to provide ammunition to pet owners whose dogs and cats have become ill after eating various brands of pet food. One of the latest such incidents is an outbreak of illness among pets who have eaten NUTRO products.

Scores of pets from California to South Carolina have experienced sudden and recurring bouts of diarrhea, vomiting, and other digestive problems after being fed with NUTRO feeds. The company denies there is any problem and says its products are safe.

More about pets ...