This graphic from the New York Times shows how one man, a Chinese medical professor who had been studying SARS has infected 249 people (As of March 26).
He stayed at a hotel for ONE NIGHT in Hong Kong, infecting other hotel guests. Two of those guests went to hospitals in Hong Kong and infected medical personnel. Other guests returned to their home countries and it began to spread them there, begining outbreaks in Canada, Singapore, and Vietnam. All from one man… Scary.