Imagine you’re a young soldier in the late 1800s, assigned to Fort Davis, a military post located in far West Texas. With a life expectancy of only 48 years (33 if you’re African American), you’ve got little spare time to ponder the inevitable. Besides, you’re too busy tending to the basics of military life—practicing drills, hauling water, taking care of the cavalry horses and other livestock, cutting wood for heat, escorting citizens across the Big Bend country, or fending off attacks by unfriendly Comanches. With little time left for anything else (except maybe an occasional binge at the local tavern), you’re definitely not sweating the small stuff. But however arduous and uncertain your life may be, it’s the small stuff that constitutes your greatest danger in the form of invisible germs, present everywhere in your unsanitary surroundings.