Imagine the workplace during flu season. Some people get sick and display clear symptoms—a warning sign to coworkers to avoid contact and for that individual to stay home. Others are infected, but never or only belatedly exhibit the tell-tale signs of sickness, meaning they can infect coworkers without knowing it. If healthcare professionals had the ability to test in advance whether a person is likely to spread a disease following infection, they could recommend specific measures to treat the person or limit exposure and perhaps keep an outbreak from growing into an epidemic or pandemic.