Clostridioides difficile (C. diff) is a stealthy threat. It infects more than 500,000 people in the United States each year, and kills up to 30,000. It is a leading cause of health-care-associated infections, particularly in hospitals and long-term care facilities. But not everyone who harbors C. diff gets sick—as many as 30–40% of the population carries this bug in their guts.