When you combine a cellphone's proximity to your ears, nose and mouth with its bacteria-loving warmth, the result can be harmful to your health. This hazard, says Jeffrey Cain, the president of the American Academy of Family Physicians and chief of family medicine at Children's Hospital Colorado, often goes unnoticed. "Some things we think are personal are actually more public than we imagine." Bacteria and other infectious agents on a phone can cause flu, pinkeye or diarrhea, says Dr. Cain.

For people who want to keep a clean touch screen, there is a disconnect between what doctors and medical researchers advise and what device makers suggest for phone sanitizing.

While products are marketed specifically for mobile-phone cleaning, they can sometimes damage the phone's screen coating or fail to remove 100% of the germs.

Even though computers, keys, pens and landline phones can harbor germs, many people's cellphones have become extra appendages—never far from the pillow, health club treadmill or restaurant table.

"We're feeding the little creatures," says Michael Schmidt, a professor and vice chairman of microbiology and immunology at the Medical University of South Carolina. "We've all seen that greasy smear [on the touch screen]. Where there is grease, there are bugs."