“Think about that the next time you place your purse or knapsack on the bathroom floor and then bring it home and put it on the kitchen table or counter,” she said.

Consider what the dog drags in

If you are concerned about what two-legged residents track in, then what about your dogs?

“We don’t wash the dog’s paws every time he comes in the house, and I don’t want to think about where he’s been walking,” said Dr. Carroll, who has a Cavalier King Charles spaniel named Lok i .

Andrea Kaufmann of Cape May Court House, N.J. , said she changed out of her shoes into slippers to keep dirt off the floors, but added that she has two Labrador retrievers.

“I could sweep and vacuum three times a day and still have dirt on the floors from the dogs,” she said. “They can’t take their shoes off.”

Dirt can be healthy. Really.

Considering the benefits of modern-day sanitation, vaccinations and health care, the likelihood of getting sick from our shoes is “infinitesimally small as to almost be unwarranted,” said Jack A. Gilbert, a professor in the department of pediatrics and Scripps Institution of Oceanography at the University of California, San Diego .

Mr. Gilbert, an author of the book “Dirt Is Good,” said there were theories suggesting that bringing elements of the outdoors indoors could help stimulate immune systems, particularly in children.

In the first year of life, physical interaction with a dog can reduce a child’s likelihood of developing asthma by 13 percent, while interactions in a barn or farm can reduce it by 50 percent, he said.