When was the last time you washed your coffee mug at the office, or water bottle, or your filtered water pitcher? Kris Frieswick writes in the Wall Street Journal, “I was bred to believe that product expiration or “best by” dates are just marketing—and marketing is for rubes. When the light on top of my water pitcher insists I change the carbon filter, I use the same approach I have used for expiration dates on prescription drugs, eggs and canned soup: I ignore it. I only swap it out when the water starts to taste funky, or when it filters too slowly, which is annoying and wastes my precious waning time on Earth.

The mold and bacteria growing in my pitcher, however, were the least of my concerns, according to Caitlin Proctor, an assistant professor of engineering at Purdue University, who studies “the entire microbial ecology of what’s growing” in drinking water systems. “There’s a whole ecosystem in there,” she says.

Most of the residents of this ecosystem don’t hurt you if ingested, she says. But carbon filters don’t kill or filter some types of opportunistic pathogens, such as Legionella pneumophila (which causes Legionnaire’s Disease) and Naegleria fowleri (better known as braining-eating amoebas). And the slimy biofilm that clings to the inside of your pitcher when you don’t wash it enough will give them a nice place to grow.

The good news, says Proctor, is that these scary bad guys don’t generally hurt you by drinking them. They can ruin your day if they are aspirated, come into contact with your eyes or get near your brain.”

What is contaminating our soul that is flying under your radar? We are not to let the contaminants of the world conform us into their “mold.”