(Wall Street Journal) — They lurk on the kitchen sponge, your computer keyboard and the dirty laundry. Flush the toilet and they become airborne. Strangers leave them behind on airplanes, gas pumps, shopping carts, coffeeshop counters and elevator buttons. Your desktop, office microwave handles, and the exercise bike at the gym are covered with them. Don’t even think about the toys at day-care centers or the kids’ playground equipment.

Germs—the microscopic bacteria, viruses, fungi and protozoa that can cause disease—cling to the most common surfaces and then hitch a ride on our hands. As swine flu spreads from person to person around the world, it is most often being transmitted by coughing or sneezing, but it can also infect people who touch something with flu virus on it and then touch their mouth or nose, the Centers for Disease Control and Prevention warns. And like an unwelcome house guest, a flu virus can hang around for days.

No wonder germophobes—including me—are on high alert, viewing every surface as a potentially lethal petri dish. We’re using our elbows to push elevator buttons, forgoing the handshake and social kiss for the fist bump, and fanatically disinfecting everything in sight.

But how vulnerable are we to the sea of germs swirling around us? Our immune system protects us from most of them, and in some spots that harbor germs, like household drains, the risk of transfer is low. Experts say there’s no reason to panic—even though there may be good reasons to be grossed out, since the spread of germs is often linked to poor bathroom hygiene and bacteria from human waste.

Cleaning and disinfecting things like desks and doorknobs can play a role in protecting us, he says, but “focusing on one surface misses the point, because no surface is not germy.” (The CDC.gov Web site offers information on keeping germs at bay in the home, how to wash your hands correctly, and the importance of flu vaccines and other immunizations in preventing disease.)Also, not all germs are harmful; we need friendly bacteria that live on our skin to help fight off bad bugs, and bacteria in our mouth and gut help digest our food and prevent illness and disease.

Still, I wanted to know where in my home, office and wider world I should most forcefully brandish my disinfectant wipes and hand-sanitizer. My calls to experts turned up some surprising culprits: the public toilet seats I’d always been warned about are likely cleaner than the desks in my workplace. My kitchen sponge and cutting board harbor the biggest dangers, as do places like elevator buttons, communal coffee carafes and gym equipment, that are touched by many hands and are rarely cleaned.

One of the scariest germ incubators may be the office. Your co-worker eating at the next cubicle isn’t just annoying you with the smell of fried onions—he’s leaving behind particles of food that can be breeding ground for bacteria. Add in the microbes transferred from workers’ hands to keyboards, phones and the computer mouse, and the average office desk is may harbor 400 times more germs than the average toilet seat, since office desks and surfaces may be rarely cleaned, while bathrooms tend to be disinfected regularly, Dr. Gerba says.

After testing surfaces and objects in 113 offices in five cities, the Arizona researchers found that women’s offices had more than twice the bacteria of their male counterparts. Makeup cases, phones and purses had the highest number of bacteria; for men it was wallets, hand-held electronic devices and phones. Women’s offices had higher numbers of mold and yeast, mostly from food kept in drawers. But the superbug MRSA, isolated in 6% of offices, was found more often in men’s offices on the phone, computer mouse, desktop and the bottom of desk drawers.

The studies are funded by makers of disinfectants including Procter & Gamble and Clorox, whose products were also used to test the effectiveness of cleaning and compare regular cleaning regimens to disinfecting with substances like bleach. Dr. Gerba says more research is needed on the link between surface germs and disease, since it’s impossible to say who will get sick. “Some people will never get ill no matter what they do or don’t do, and others will get ill almost every time,” he notes.

A washable keyboard and mouse are a great way to minimize the amount of germs that linger on your desk. WETKEYS.com offers a wide selection of cleanable keyboards and mice for any work environment and keyboard user.

Where the Worst Germs Lurk