If a kitchen represents a temperate forest, few of its plants would be poison ivy. Most of the inhabitants are relatively benign. In any event, eradicating them is neither possible nor desirable. Dr. Fierer wants to make visible this intrinsic, if unseen, aspect of everyday life. “For a lot of the general public, they don’t care what’s in soil,” he said. “People care more about what’s on their pillowcase.” (Spoiler alert: The microbes living on your pillowcase are not all that different from those living on your toilet seat. Both surfaces come in regular contact with exposed skin.)

Dr. Fierer has teamed up with Rob Dunn, a biologist at North Carolina State University, to sample the microbial wildlife in 1,400 homes across the United States. The project — known as The Wild Life of Our Home — relies on volunteers who swab pillowcases, cutting boards and doorjambs, then send samples in for analysis. (Full disclosure: I am a volunteer participant in the study, sending in swabs from my home.) Dr. Dunn hopes that the project will begin to unravel the consequences of moving from caves to creating environments around us in a haphazard way.

“For the entire history of humanity, we have created environments around us, in our daily lives, in a very unintentional way. The control that we’ve exerted is predominantly one in which we kill the ones that might be bad,” Dr. Dunn said. “That’s saved a lot of lives. It’s also favored this whole suite of species that we know very, very little about.”

So little is known of what’s in the home that a small sample can reveal something new. In their first study of 40 homes around Raleigh-Durham, N.C., published last week in the journal PLoS One, they found that humans rapidly “infect” the spaces in which they live. We leave bacteria by touching surfaces with our exposed skin, and at room temperature, a healthy human kicks up a “convective plume” of about 37 million bacteria per minute that disperse throughout the home and can survive for extended periods.

Species living outdoors find their way indoors, Dr. Fierer and Dr. Dunn found. Outdoor microbes are more frequent in homes with dogs, where fur-associated bacteria were found adhering to TV screens and pillowcases. “My expectation is that the effect of dogs on allergies is similar to the effect of getting your kid playing in the dirt,” Dr. Dunn said. “They end up being a surrogate for dirty nature in some ways, one dirty paw at a time.”

The data from the 1,400 homes are still being analyzed, but the team expects to learn how building materials, ventilation rates and cleaning habits affect the microbial map. “If you’re a vegetarian, do you have different microbes in your house than if you’re an omnivore?” Dr. Fierer said. “If you live in a forest, do you have different microbes than if you live in a desert? These are the types of questions that we don’t actually know the answer to. Yet.”