According to an idea called the hygiene hypothesis, exposure to germs and certain infections — especially at a young age — actually helps prime the immune system so it can defeat these microbes more easily in the future.
Some research has suggested that the idea could partially explain why children who grow up around animals and in rural areas appear to develop conditions like asthma less often than children who don't. More studies, however, are necessary.
And even for people other than young children, the hygiene hypothesis makes intuitive sense: After all, literally every surface in the world is covered in bacteria. The idea that things can be "perfectly clean" is a myth — humans need bacteria to live.
"We tend to think of our homes and personal environments as these pristine places, and public ones as dirty and infested with bacteria," Chris Mason, a Weill Cornell Medical College geneticist and the author of the subway-pathogen study, recently said at a public event in New York. "But you should really think of yourself as a rabbit who gets to hop between two forests."
That's why Mason isn't afraid to let his own young daughter ride the subway or play in the dirt.
"I would advise any new parent to roll their child on the floor of the New York subway," said Mason.
Like the surfaces people touch and the ground they walk on, the human body is already teeming with thousands of different species of bacteria, from the Lactobacillus acidophilus lining digestive tracts to the Propionibacterium acnes populating the skin on faces and arms. On average, about three pounds of our body weight is accounted for by bacteria alone.
So the idea that a little more exposure couldn't hurt makes sense. Perhaps everyone should be a little less germaphobic.