the so-called basic reproduction number (also called basic reproduction ratio). This ratio is derived as the expected number of new infections (these new infections are sometimes called secondary infections) from a single infection in a population where all subjects are susceptible. This idea can probably be more readily seen if we say that the typical time between contacts is formula_10, and the typical time until removal is formula_11. From here it follows that, on average, the number of contacts by an infectious individual with others "before" the infectious has been removed is: formula_12