When scientists are faced with a new epidemic, they must act to understand how quickly it spreads. They do this by estimating the basic reproduction number called the R0, or R nought, which reflects how infectious a virus is. It is defined as the average number of people an infected person can expect to pass the virus on to before any widespread immunity or attempts at immunisation are made. For example, if one person develops an infection and transmits it to two other people, the number would be R2. The higher the number, the faster it progresses.