One of the key features of an infectious disease is that it can be transmitted from one individual to another, leading to a chain of new disease cases. The first case in a transmission chain is often known as the primary 'index' case, with subsequent secondary person-to-person transmission. Epidemiologists often try to reconstruct the chain of transmission that occurred during an outbreak to understand the patterns of spread of the disease, measure the degree of infectiousness or transmissibility, and ultimately to predict and plan for the potential number of victims that could be expected in the outbreak or future outbreaks.

To quantify the degree of infectiousness or transmissibility, epidemiologists use a few key measures to describe the number of new cases. These include the basic reproduction number, the net reproduction number, and the attack rate.

Basic reproduction number (denoted ‘R0’) – this is the average number of new (secondary) cases, or successful transmissions, per case in a totally susceptible population.