Keeping rats as pets can come with the stigma that rats supposedly transmit dangerous diseases to their owners. Usually, rats bred as pets are tested and treated for diseases and parasites. One fear is that all rats carry plague, when in fact "R. norvegicus" is not among the list of species considered a threat. In 2004, an outbreak of salmonella in the United States was connected to people who owned pet rats. However, it has been determined that a pet rat's initial exposure to salmonella, along with many other zoonotic rat diseases, typically indicates exposure to "wild" rodent populations, either from an infestation in the owner's home, or from the pet's contaminated food, water, or bedding.