Cell phone provider AT&T is issuing a warning to its customers about the flu: the mobile devices can spread the flu virus easily.

That's because of how often people shake hands, turn doorknobs, and use railings before using their phones.

"If you don't share a cell phone, you don't have to worry because it's only your germs," AT&T's Chief Medical Officer Geeta Nayyar said.

"But if one person has the flu virus on their hands, it will be on the hands of 40 percent of the other people in the office within four hours," she warned.

Chuck Gerba, a professor of microbiology at the University of Arizona, agreed.

"Cell phones are 10 times as dirty as a toilet seat," he told ABC News.

Nayyar advised people clean and disinfect their phones regularly, use hands-free headsets, and avoid taking their phones into restrooms.