A few months before SARS sparked major outbreaks in several countries in early 2003, there were a series of small clusters of infection in Guangdong province, in southern China. Between November 2002 and January 2003, seven clusters were reported in Guangdong, ranging from one to nine cases in size. Based on the scope of these outbreaks, researchers later estimated that the reproduction number—R, which is defined as the average number of new infectious caused by a typical case—may have been around 0.8 during this period. However, by the time the first major outbreak occurred in Hong Kong a couple of months later, SARS had an R of more than 2.
There are several reasons the R of an infection may increase, and we can understand them better by breaking transmission down into four main components. Consider the steps that lead an infectious person to spread the infection to others. It ...
