Mucus is our friend. It’s the reason we don’t always get sick when someone sneezes on us; it traps viruses before they wreak havoc.

Some bugs, though, don’t get trapped. They infect us, and so we devise other ways to combat them. Most research dollars go toward that cause. But biomedical engineer Sam Lai is taking a step back. He wants to lend a little help to our friend.

“Mucus continues to be overlooked in infectious diseases,” Lai says. “For instance, it wasn’t even known how HIV penetrates vaginal secretions to establish infection until almost thirty years into the pandemic.”

Lai studies how mucus fails to stop viruses and what can be done about it.

At the molecular level, mucus is a dense network of fibers called mucins that create a three-dimensional sticky mesh. “Similar to how a spiderweb can trap a variety of bugs,” Lai says, “mucus can trap pathogens that are either larger than the pores in the mesh or that adhere to the mesh fibers.” HIV is one of many viruses that can slip through the mesh and infect the body. If we could trap HIV in mucus, Lai says, then the mucus’s natural clearance and degradative mechanisms would take care of the rest. “Most viruses die very quickly if they don’t reach the cells they want to infect,” he says.

In one experiment, Lai mixed HIV into normal cervicovaginal mucus and found that the virus could not break through the mucus. Lai thought that was because the mucus was naturally acidic. During sexual transmission, though, semen neutralizes the pH of cervicovaginal mucus. So in another experiment Lai altered the mucus so it was not acidic, and he found that HIV readily broke through the mucus, just as it does during intercourse.

But Lai realized that lactobacilli—bacteria found naturally in the vaginal tract—secrete lactic acid. That’s what caused the mucus to trap HIV as long as semen was not present. “Our results tell us that if we can maintain the acidity of the vaginal tract with lactic acid during intercourse, we can likely trap the virus,” Lai says.

Lai’s unique take on strengthening our natural defense against infectious diseases earned him a $100,000 Gates Foundation Grand Challenges Explorations grant. If he shows progress at the end of a year, that amount could increase tenfold.

“Fighting viruses after they have reached their targets for infection is like trying to defend a castle by locking the interior doors but leaving the gate open,” Lai says. “We could fend off viruses much better if we could just close the front gate.”