By Zahra Barnes

There are two kinds of people in this world: Those who keep their shoes on at home and those who scream internally at the very thought. If you don’t take your shoes off indoors, you might just not see the need. Germs are everywhere, right? But if you’re an adamant shoes-off person, your skin might crawl at the prospect of tracking all that...stuff...from the ground outside into your place. Here, germ experts discuss how important it actually is to take your shoes off as soon as you cross your home’s threshold.

First things first: While it’s true that germs are all over the place, your shoes can be particularly unhygienic.

The word “germ” often seems synonymous with “invisible particle that wants to get you sick,” but they’re not one and the same. “Germs are all over us,” says Rachael Lee, M.D., assistant professor in the University of Alabama at Birmingham’s division of infectious diseases. “They live on everything.” Simply being around germs isn’t enough to make you ill, Dr. Lee says. If it were, everyone would be sick all the time.

With that said, things get a bit grosser when you think about what your shoes might be touching all day long. If you live in a city, the sidewalks can be covered in everything from globs of spit to bird droppings. If you’re in a rural area, well, birds probably still live there—along with other animals that have no compunction when it comes to pooping all over the place.

No matter where you live, if you use public bathrooms, your shoes are treading on—and potentially picking up—all sorts of microbes. “Just because it looks physically clean doesn’t necessarily mean it’s sterile,” says Meghan A. May, Ph.D., associate professor of microbiology and infectious diseases at the University of New England College of Medicine. “Unless you see somebody in a public bathroom mopping the floor with bleach, chances are there’s something interesting on the floor that you might not want in your house.”

Yup, various microbes can indeed hitch a ride home on your shoes. May points to a small 2015 study in Microbiome that sampled the microbial environment on people’s shoes after they’d walked in various locations. “What they found was basically that when you walk on surfaces, you walk on microbes that wind up on your shoes,” she explains. “The opposite is also true: What’s on your shoes winds up rubbing off on the surfaces you walk on.”

Cool, cool, just trying to process this foul information. But can all of this actually make people sick?

Maybe. Before delving into why, it’s important to remember that not all germs are actual pathogens, aka disease-causing microorganisms. Translation: Simply tracking germs into your home doesn’t spell disaster for your health. It really depends on the pathogen, people, and home in question.

“Some microbes are much more environmentally stable than others,” May says.

Take norovirus for example. It can cause gastroenteritis (i.e., irritation and inflammation in the stomach or intestines) that leads to symptoms like diarrhea, vomiting, nausea, stomach pain, fever, headache, and body aches, according to the Centers for Disease Control and Prevention. As the leading cause of gastroenteritis in the United States, norovirus can get all over bathrooms when people poop or vomit it out. Unfortunately, with its potential to stay on surfaces for days or weeks, norovirus can also be more committed to survival than early aughts Destiny’s Child.

OK, you might be thinking. I might get something like norovirus on my shoes, but it’s not like I lick my boots every night. How would that actually make me sick? Well, it might not.

“If you are young and healthy and you clean your house once a week, I think you’re going to be fine,” Dr. Lee says. But if you live with children, you’ll want to be aware of this potential threat. “Little kids are in much more intimate contact with the floor than, say, a couple in their mid-20s,” May says. “If you happen to collect any viral particles on your shoes, deposit them on your floor, and have a 2-year-old playing on the floor, that becomes a very plausible scenario for how infection could occur.”

This is also something to bear in mind if there are immunocompromised people in your home, like someone who’s on chemotherapy or is elderly. “They are much more likely to be in a dangerous spot because of their immune system,” May says. “The average person might be able to ingest a certain number of pathogens without getting sick, but in an immunocompromised person, that number’s going to be very, very low.” Even in this kind of instance, keeping your shoes on inside doesn’t immediately mean an immunocompromised person will get sick, just that there could be a higher risk.

This effect may be worse if you have carpeting instead of hard flooring for a few reasons, May says. First, pathogens typically do best in damp environments, and it’s easier for hard surfaces to remain dry than it is for a carpet. Plus, carpets tend to be harder to disinfect than hard flooring options. Finally, carpets can act almost like a bristle, pulling microbes from your shoes as you walk.

So, when it comes to making your own shoes-on-or-shoes-off policy, it really comes down to how comfortable you are potentially tracking in illness-causing germs.

Perhaps you have kids, in which case you might feel safer kicking your shoes off at the door and using designated indoor slippers, Dr. Lee says. Or maybe you’re living in a house full of young adults and couldn’t care less about outdoor germs joining the party. Whichever side you’re on, now you’ve got the expert-provided insight to put your foot down.