In a report published Wednesday, Chinese researchers suggested that person-to-person spread among close contacts occurred as early as mid-December. Based on the first 425 confirmed cases, the researchers estimate that each infection led to 2.2 others on average. That’s a bit more than ordinary flu but far less than some other respiratory diseases such as whooping cough and tuberculosis. The rate for SARS, a cousin to this new virus, was estimated to be 3.