Final answer:
The rate describing the number of new cases of an illness for a specific time period is the incidence rate, which is different from prevalence, mortality, and attack rates.
Step-by-step explanation:
The rate that describes the number of new cases of an illness for a specific time period is the incidence rate. Unlike prevalence rate, which includes all cases (new and existing) within a population over a period of time, the incidence rate specifically measures the new cases, thereby helping to understand the risk and spread of diseases.
The mortality rate is concerned with the number of deaths attributable to a disease, and the attack rate usually refers to the proportion of people who become ill in a population after exposure to an infectious agent.