The Centers for Disease Control and Prevention says dozens of people have been sickened with infections linked to puppies sold at Petland pet stores.

The outbreak of Campylobacter has affected 55 people in 12 states. Thirteen people have been hospitalized. This is an increase from the 39 cases reported in seven states last month.

Campylobacter infections can be spread by contact with dog feces.

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"If you don't wash your hands afterwards, innocently touching your mouth or touching something that comes in contact with your mouth, you're able to get it," Dr. Brian Lamb, from West Penn Medical Associates, told CBS Pittsburgh.

He says Campylobacter infections are not uncommon, and that many people have no symptoms at all.

However, there are certain symptoms people should watch for.

"Diarrhea is going to be the number one symptom, as well as abdominal pain and cramping," Lamb said. "Usually about two to five days after you've been exposed to it."

Fifty-five people in 12 states have been affected by a Campylobacter outbreak tied to puppies sold at pet stores. istockphoto

Dr. Kristen Mertz, a medical epidemiologist for the Allegheny County Health Department in Pennsylvania, where two cases have been reported, said in a written statement when the outbreak first emerged in September: "This is not something new in our region. We see frequent cases of Campylobacter associated with puppies."

Dr. Mark Laughlin, a veterinary medical officer at the CDC, told CBS News hand-washing is the best defense.

"It's important to wash your hands, and wash your hands thoroughly with soap and running water for 20 seconds any time you come into contact with animals. Especially when you are cleaning up animal poop or you come into contact with their food or their environments," he said.

Petland says it is working with the CDC to address the outbreak and is continuing to educate staff and consumers about proper hand sanitizing.