One of the most useful quantities for predicting disease spread is a variable called the basic reproduction number. Usually designated as R0, it measures the number of uninfected people that can expect to catch a disease from a single infected individual. An R0 of 4, for instance, means that—in the absence of vaccines or other preventative measures—someone who gets a disease can be expected to pass it along to four other individuals before recovering (or dying) from it.
