Five cases of the mysterious Wuhan coronavirus have been confirmed in the United States, giving rise to concerns about a potential global pandemic. We’ve seen this story before, as health authorities working with threadbare data try to walk the line between epidemic readiness and needless panic. Is this new outbreak poised to become the next AIDS pandemic or a new SARS, which was stopped in its tracks after 774 deaths? To cut through the headlines, we can use a simple concept called the “epidemic triangle.” Employed by epidemiologists since the discipline’s earliest days, it is indispensable in predicting whether localized outbreaks will transform into full-blown epidemics.