To be clear, SARS-CoV-2 is not the flu. It causes a disease with different symptoms, spreads and kills more readily, and belongs to a completely different family of viruses. This family, the coronaviruses, includes just six other members that infect humans. Four of them—OC43, HKU1, NL63, and 229E—have been gently annoying humans for more than a century, causing a third of common colds. The other two—MERS and SARS (or “SARS-classic,” as some virologists have started calling it)—both cause far more severe disease. Why was this seventh coronavirus the one to go pandemic? Suddenly, what we do know about coronaviruses becomes a matter of international concern.