There’s more than just the ick factor when it comes to using a public toilet. There’s also the concern over what nasty, disease-causing germs you can catch from the seat or even the lock that lets you out of the stall.

Since it's virtually impossible to avoid public toilets for the rest of your life, we asked University of Arizona germ expert Chuck Gerba, PhD, to fill us in on what malicious microbes are likely to be lurking on public toilets and what they might do to us.

On one thing the germ gurus agree: You are not going to catch a sexually transmitted disease (STD) from a public toilet. So much for that worry.

But that doesn’t mean toilet seat germs can’t make you sick — in theory. You could pick up enough organisms to experience gastrointestinal distress or to vomit, but the germ load would in most cases have to be substantial, and your immune system and hand washing habits not up to par, says Gerba

Gerba summarizes what to worry about this way: "Basically, it's diarrhea, not gonorrhea."

Related: How Not To Bring Germs Home with You

Common bugs in public loos

The germs that have been shown to be transmitted by public toilets, Gerba says, are:

  • Norovirus, aka the “cruise ship” bug that causes vomiting
  • MRSA (methicillin-resistant Staphylococcus aureus), the drug-resistant staph infection bug
  • Shigella, which causes a nasty intestinal disease involving diarrhea and fever
  • Hepatitis A, which causes liver infection
  • Salmonella, the bacterium you know best as a cause of food poisoning (and diarrhea, fever and cramping)

Toilet germ hotspots: cruise ships, hospitals and airplanes

Some places are more likely to transmit certain infections than others, Gerba says. For instance, norovirus is often found on cruise ships. And it is highly infectious. "It takes just one to 10 viruses to cause infection," he says. That dose may be enough to give you diarrhea or projectile vomiting, Gerba says.

In one study, MRSA was recovered from about 3 percent of hospital toilets when alcohol wipes were not used. MRSA can cause skin infections.

But from Gerba's research, he concludes that "the worst toilets are airplane toilets.'' That's due to heavy use, he says.

On a typical plane, 50 people can be using a bathroom on a flight. On some planes, 75 passengers may share a single bathroom. "So they get pretty germy," he says. The automatic faucet shutoff that gives passengers just a brief burst of water often means hands may not get washed thoroughly, he says.

Next germiest, he says, are the toilets in airports, then trains, then buses.

Related: Having Surgery? 8 Ways to Avoid a Hospital Infection

Protecting yourself

Will those paper seat guards help guard you from germs? Gerba says there's no evidence they work, although they may make people feel more comfortable. He's not aware of any research showing those plastic toilet seat barriers that move a fresh cover into place before each use work, either.

Alcohol wipes, used on hospital toilet seats, reduced the average daily bacterial counts by half, according to some research, and eliminated MRSA.

What else can you do? Washing your hands and making a “clean” break from the restroom without contaminating them again is key. That's not so simple, though, considering Gerba found in his research that sink faucets are germ-ridden. (So are toilet flush handles, but at least you'll be washing your hands after touching them.)

Ideally, he says, the best way to reduce germ exposure is to visit a bathroom with an automatic flush toilet, a touchless sink faucet (one that turns on with a wave of your hand) and automatic towel dispensers.

Related: Hand Dryers Spew Bacteria, But Should You Worry?

Otherwise, be sure to wash your hands with soap and water for at least 20 seconds. (Sing the happy birthday song twice, start to finish, to time yourself, the Centers for Disease Control and Prevention suggests). Avoid touching the faucet with your bare hands (use your elbow or a paper towel). Air or towel-dry your hands. Use a paper towel if you have to touch the door handle or latch on the way out.

If there no water around (say you're using a porta potty), the CDC recommends using an alcohol-based hand sanitizer that is 60 percent alcohol.

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Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.