What is defined as the frequency of new cases of a disease or condition in a population during a limited time period?

• A. Prevalence
• B. Incidence
• C. Attack rate
• D. Mortality rate

Answer: (C)

When a cluster or outbreak is investigated, the measure you use to describe how many people got sick during a specific, short time window is the attack rate. It focuses on new cases that occur among those who were exposed during that defined period, giving a sense of how contagious or transmissible the situation was within that brief window. You calculate it by taking the number of new cases during the outbreak and dividing by the number of people who were at risk or exposed, often converting to a percentage. This is a time-bounded cousin of incidence, tailored for outbreaks where there’s a clear exposure period and rapid onset.

This differs from incidence in a broader sense, which looks at new cases in a population at risk over a period but isn’t limited to a specific exposure event. Prevalence, on the other hand, counts all existing cases (new and preexisting) at a point or over a period and isn’t about new occurrences during a defined time. Mortality rate is about deaths, not new disease occurrences. For an outbreak with a defined exposure period, the attack rate best captures the concept of new cases in that limited time.