Millions of Americans climb into buses, subway cars and planes every day, but do they know what's climbing in with them?
"You should probably take some precautions," germ expert and University of Arizona microbiologist Dr. Chuck Gerba said. "You never know who is the last person who grabbed that pole or boarded that bus."
According to Gerba, when it comes to public transportation, some of the "germiest" spots are places you might not expect.
To find out what kinds of germs are lurking on the armrest or flush handle, ABC News sent Gerba on a road trip and an airplane trip.
He tested dozens of the surfaces people touch every time they travel.
Starting with the buses, Gerba said of all the restrooms he tested, the one on a Greyhound bus was the germiest. He found tens of millions of E. coli bacteria on one toilet seat.
"Finding E. coli indicates there's likely fecal matter present," Gerba said. "And if that's so, there's other types of bacteria and viruses that can be present that can cause diarrhea, other types of infections, hepatitis."
Gerba was surprised to find relatively low levels of bacteria on the New York City subway. But he did find elevated levels of mold, a potential problem for people with allergies.
He was not prepared for what he found on an Amtrak train -- coliform levels in the millions on restroom sink handles. Coliform is a bacteria that can indicate fecal contamination.
He also found high bacteria levels on café car tables, and MRSA, a bacteria that can cause skin infections, on the armrests.
"We always found large numbers of bacteria on Amtrak surfaces," he said. "Usually we were finding maybe 100,000 bacteria per square inch."
Next, Gerba collected germ samples on four flights. With a lower volume of passengers, he expected the airplanes to be the least germy mode of transportation of all; they were not.
"Some planes were fairly clean and some planes weren't," he said. "But surprisingly, almost all the food trays you pulled down had large numbers of bacteria on them."
Inside the airplane restrooms, Gerba found E. coli on the toilet seat, the sink, and the flush handle.
On all the planes trains and buses, Gerba found high bacteria levels on spots that are tough for cleaners to reach, like luggage racks. He said hand sanitizer should be a must for regular commuters.
"Remember, you're always gambling with germs," he said. "The whole idea here is not to become over-paranoid, but to keep the odds in your favor all the time."
The companies and associations involved in the investigations made the following statements to ABC News:
Metropolitan Transportation Authority, State of New York
During rush hours, train cars are swept and spot mopped at terminals and a more thorough mopping is done during non-rush hours. During late night hours, trains are mopped and seats are wiped down. When a train goes in for routine maintenance (every 66 days or 10,000 miles, whichever comes first) the car's interior is thoroughly cleaned, from floor to light lenses.