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Case of Incidence and Prevalence

Scenario No. 1

Out of the 75 persons who attended the picnic, 46 subsequently developed gastroenteritis.

  1. Calculate the attack rate?
  2. Draw an appropriate epidemic curve.
  3. Is this a primary or a secondary attack rate?
  4. Is this an incidence of prevalence rate?
Scenario no. 2:

7 cases of hepatitis A occurred among 70 children attending a child day care center. Each infected child came from a different family. The total number of persons in the 7 affected families was 32. One incubation period later, 5 family member of the 7 infected children also developed hepatitis.

  1. Calculate the attack rate.
  2. Draw the epidemic curve.
  3. Is this primary of secondary attack rate?
Scenario no. 3:

The results of a study of the incidence of pulmonary tuberculosis in a village in India and given in the table below. All the persons in the village are examined during the surveys made 2 years apart, and the number of new cases was used to determine the incidence rate.

Category of Households at first survey Number of  persons Number of new cases
With culture positive case 500 10
Without culture positive case 10,000 10

What is the incidence of new cases per 1000 person years in households had a culture positive case during the first survey?


By the end of the session, the students will be able to:

  1. Differentiate between Rate, Ratio and Proportion.
  2. Differentiate between Incidence and Prevalence of a disease.
  3. Appreciate various factors affecting Incidence and Prevalence rates.
  4. Identify different types of Incidence and Prevalence rates.
  5. Draw different epidemic curves.

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