2. Prevalence: number of cases at a given time expressed as a percent at a given time. Prevalence is a product of incidence x duration of disease, and is of little interest if an infectious disease is of short duration (i.e. measles), but may be of interest if an infectious disease is of long duration (i.e. chronic hepatitis B).

3. Attack rate: proportion of non-immune exposed individuals who become clinically ill.

4. Primary/secondary cases: The person who comes into and infects a population is the primary case. Those who subsequently contract the infection are secondary cases. Further spread is described as "waves" or "generations".

5. Case fatality: proportion of infected individuals who die of the infection. This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed.