In outbreak investigations, which rate links illness to exposure to a food item?

• A. Food-specific attack rate
• B. Baseline rate
• C. Incidence
• D. Mortality

Answer: (A)

Attack rates help investigators determine if illness is tied to a specific exposure. The food-specific attack rate is the measure that links illness to exposure to a food item. It is calculated by dividing the number of people who became ill after eating that food by the number of people who ate that food. For example, if 15 of 20 people who ate potato salad fell ill, while only 1 of 30 who did not eat it became ill, the food-specific attack rate among eaters is 75% and the rate among non-eaters is about 3%. A much higher rate among those who ate the food supports that the food item is the source. This concept differs from incidence, which is new cases in a population over time regardless of exposure; a baseline rate, the usual level of illness in a population; and mortality, which is the number of deaths.