As flowers bloom and temperatures climb, many are eager to get back outside. But while the Sun may be shining, there is a dark side that can make the great outdoors not so great.

Gangs of germs are lurking in the woods, in the soil, in the water and in your food, ready to rain on your summer parade.

I’m a professor of microbiology^[1] at the Indiana University School of Medicine, where I study and teach about...