A few years ago, a Belgian man in his 30s drove into a lamppost. Twice. Local authorities found that his blood alcohol level was four times the legal limit. Over the space of a few years, the man was apprehended for drunk driving three times. And on all three occasions, he insisted he hadn’t been drinking.

He was telling the truth. A doctor later diagnosed auto-brewery syndrome—a rare condition in which the body makes its own alcohol. Microbes living inside the man’s body were fermenting the carbohydrates in his diet to create ethanol. Last year, he was acquitted of drunk driving.

His case, along with several other scientific studies, raises a fascinating question for microbiology, neuroscience, and the law: How much of our behavior can we blame on our microbes?

Each of us hosts vast communities of tiny bacteria, archaea (which are a bit like bacteria), fungi, and even viruses all over our bodies. The largest collection resides in our guts, which play home to trillions of them. You have more microbial cells than human cells in your body. In some ways, we’re more microbe than human.

Microbiologists are still getting to grips with what all these microbes do. Some seem to help us break down food. Others produce chemicals that are important for our health in some way. But the picture is extremely complicated, partly because of the myriad ways microbes can interact with each other.

But they also interact with the human nervous system. Microbes can produce compounds that affect the way neurons work. They also influence the functioning of the immune system, which can have knock-on effects on the brain. And they seem to be able to communicate with the brain via the vagus nerve.

If microbes can influence our brains, could they also explain some of our behavior, including the criminal sort? Some microbiologists think so, at least in theory. “Microbes control us more than we think they do,” says Emma Allen-Vercoe, a microbiologist at the University of Guelph in Canada.

Researchers have come up with a name for applications of microbiology to criminal law: the legalome. A better understanding of how microbes influence our behavior could not only affect legal proceedings but a...