Eons ago, humans living in the wild were exposed to every germ nature could throw at them—mud, microbes, and whatever drifted around the communal waterhole. We’ve traded tree canopies for roofs, but the germs are still showing up, hiding in our household possessions.

We often overlook the dirtiest items in our homes because grime is invisible (think bacteria) or accumulates too gradually for us to notice. Another factor: our brains tend to tune out the appearance of familiar, everyday objects, in a phenomenon called attentional blindness. This means we’re least likely to scrutinize the things we touch the most, even as they become swamped by bacteria.

Some argue that chronic immune conditions that plague modern humans stem in part from our tendency to avoid beneficial germs that would train the immune system to fight off pathogens. (This is called the hygiene hypothesis.) “There’s some truth to that,” says Kelly Reynolds, professor of environmental risk at the University of Arizona. “But we do need to reduce risk and exposure to bad bacteria that can drive sickness,” she says, including colds and skin infections.

Here, experts reveal which items in your house are harboring the most bacteria—and how to clean them