Eighteen primary shigellosis cases arose in 18 households holding 86 people in all, and 17 contacts later fell ill. What is the secondary attack rate?

1. A secondary attack rate divides cases among contacts by the total number of contacts. 2. Contacts are the household population minus the primary cases: 86 minus 18 equals 68. 3. 17 divided by 68 equals 0.250. 4. Multiplying by 100 gives 25.0%. 5. Leaving the primary cases in the denominator gives 17 over 86, or 19.8%, the commonest error here. 6. 94.4% divides the secondary cases by the number of households, and 20.0% comes from subtracting only one person instead of all 18 primary cases. _Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section Properties and uses of incidence proportions_