Armed with Q-tips and test tubes, the microbiologist from the University of Arizona takes samples from places people frequent every day, and tests for dangerous germs and bacteria that can be passed on with a single touch.
48 Hours Correspondent Peter Van Sant accompanied him on his rounds.
You might say Gerba has spent a good part of his life in public restrooms, that is "professionally," he said.
"My odds of finding E. coli here are really high," Gerba said.
"We literally pick up germs that can make us ill every day in all of our activities," Gerba said. "And we're playing a Russian roulette game. And the thing is, with a little conscious effort, to try to win at Russian roulette."
She tested for bodily fluids that can carry disease. He checked for fecal bacteria and E. coli. They collected samples at a playground, a bus station and shopping mall and anywhere germs could hide, even a merry-go-round.
"It's kind of fun because it's always really like a detective story trying to find out where the germs really are," Gerba said.
And the next stop for the germ detectives: the home of the Peterson family: Kim, Chuck, and their two boys.
Kim Peterson said she thinks she has a pretty clean house. So she volunteered to put it to the test.
Gerba suggested that "probably the best place is in the kitchen."
Before he even found any bacteria, though, Gerba started to make the family feel sick. "There are actually 200 times more fecal bacteria on the average cutting board in the home than (on) the toilet seat," he said.
They tested a dishrag in the sink, the children's toys and the washing machine.
"If I ever come back in the next life as a bacteria, I'd want to be in a dishrag," Gerba said.
"Basically if you do undergarments in one load and handkerchiefs in the next, you're blowing yor nose in what was in your underwear," Gerba explained.
"It sounds kind of frightening to me; we'll have to wait and see what the results are," Kim Peterson saod/
By morning the results were in.
"The underside of the table at the picnic place was positive for bodily fluids," Reynolds reported.
Bodily fluids turned up in almost every public place.
"Mucus, saliva or urine on the high chair at the mall," Reynolds said.
And one in four test tubes turned yellow, an indication of fecal bacteria.
Then Gerba checked for E. coli. If it is present, the tubes will glow under an ultraviolet light.
Out of all the samples, there were four positives, including one from the handrail on the bus.
And where were the other three? "This was from the sink, this was from the dishrag, and this one here is from the washing machine," Reynolds said.
The Peterson's clean home was actually an E. coli hot zone. "It's a biohazard basically," said Gerba of the dishrag.
Odds are it did not carry the deadly strain of E. coli. But it could make someone sick, with diarrhea and painful stomach cramps.
So how can people protect themselves?
Clean with bleach or disinfectants, Gerba advised. Soap alone won't kill germs.
And wash your hands regularly, because dangerous germs can be in places you'd never suspect.
"If you're making a salad, and the carrot fell in the sink, you'd rinse it off and put it back in," Gerba said. "Well, there are more fecal bacteria in the sink than in your toilet. If your carrot fell in the toilet bowl, would you put it back in the salad?"