It might happen to you because you’re an adventuresome diner or traveler. Or because of where you were born, or the kind of work you do. Or because you engaged in risky sex, even long ago, or experimented with drugs, or had a blood transfusion.

Millions of unsuspecting Americans harbor virus infections in their liver. Collectively they share one name: hepatitis, but the three most common forms, which go by the letters A, B, and C, are all quite different in how you catch them and what risk they pose to long-term health...