If it’s a particularly contagious virus, it would spread across the planet in a year. “If it starts in New York, it’s going to be in London certainly within a week,” says Ira Longini, a biostatistician at the University of Washington and the Fred Hutchinson Cancer Center in Seattle who uses computer models to analyze how viruses globe-trot. “And from there, it will quickly travel to the rest of North America and Europe.” For Longini’s computer forecasts to become reality, though, certain conditions would need to be met.