E ons 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,