Attack rate as risk in an exposed group
An attack rate is the proportion of an exposed population that develops disease over a defined period, calculated as cases divided by the number exposed. It is really an incidence proportion used in acute outbreak settings, such as the share of guests who fell ill after a shared meal. Comparing attack rates across exposure groups is a standard way to point toward a likely source.
Because it is bounded between 0 and 1, an attack rate is easy to read as a risk and to compare between subgroups in an investigation.
Secondary attack rate and onward spread
The secondary attack rate measures spread from a primary case to susceptible contacts, classically within a household. It is the number of new cases among contacts divided by the number of susceptible contacts, often within one incubation period of exposure to the primary case. A higher secondary attack rate indicates more efficient person to person transmission.
This is general educational information, not medical advice. Outbreak measures are population statistics and do not predict an individual's risk; public health guidance should come from qualified authorities.