Stephen S. Morse, professor of epidemiology at the Mailman School of Public Health at Columbia University, said even if a person walks through a seemingly clean office building, shoes pick up lots of unexpected grime.
Bacteria and viruses such as E. coli and those that cause influenza may also attach themselves to shoes, while those who live in farmland or near any exposed soil, might be exposed to more harmful pathogens, the Wall Street Journal reported.
But the germs would "have to make their way into a human through a lesion on the skin, and that's a pretty far-fetched scenario," Morse told the Journal.
Mud and dirt collect in the grooves of shoe soles, especially after rain or snow, while carpets and wet surfaces might harbor fungi such as athlete's foot and the virus that leads to Plantar warts, Morse said.
However, the risk of catching anything more severe is low, whether a no-shoe policy is strictly enforced or sometimes lax, Morse said.
Homes with babies, who crawl might benefit from a no-shoe policy because children are more likely to put items found on the ground in their mouths.
Morse does offer practical reasons for keeping outdoor shoes off indoor home floors. Wood floors and other surfaces can look cleaner without shoe traffic and Morse, who admits to removing his shoes at the door, said he did so "for the sake of keeping my marriage intact."