**Question:** In an area not covered by measles immunization, the attack rate of measles is -

A. 20%
B. 50%
C. 70%
D. 90%

**Core Concept:** Attack rate in epidemiology refers to the proportion of susceptible individuals in a population who develop an infectious disease after exposure to the pathogen. In the context of measles, measles immunization refers to the administration of the measles vaccine, which provides immunity against the disease.

**Why the Correct Answer is Right:** In an area where measles vaccination is not implemented, a significant portion of the population remains susceptible to the disease. When exposed to the measles virus, these unvaccinated individuals have a high likelihood of developing the disease. Therefore, the attack rate is high.

**Why Each Wrong Option is Incorrect:**

A. 20% is too low, as it suggests that only a small fraction of the population would contract measles after exposure.
B. 50% is still too low, indicating that half of the population would develop the disease, which is not the case in unvaccinated areas.
C. 70% is close to the correct answer but still too low, suggesting a smaller proportion of the population developing measles than in reality.
D. 90% indicates a high attack rate, which is accurate for an unvaccinated area.

**Clinical Pearl:** Immunization is an essential public health measure to control infectious diseases like measles. By vaccinating a significant proportion of a community, herd immunity is achieved, where the spread of the disease is suppressed due to the reduced number of susceptible individuals. In areas with low vaccination rates, the attack rate remains high, putting both vaccinated and unvaxinated individuals at risk.

**Correct Answer:** D. 90%

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In an area where measles immunization is not implemented, a significant portion of the population remains susceptible to the disease. When exposed to the measles virus, these unvaccinated individuals have a high likelihood of developing the disease. Therefore, the attack rate is high, which is accurately represented by option D (90%).

**Why Each Wrong Option is Incorrect**:

A. A 20% attack rate suggests that only a small fraction of the population would develop the disease, which is not the case in unvaccinated areas.
B. A 50% attack rate is too low, indicating that half of the population would develop measles, which is not the situation in unvaccinated areas.
C. A 70% attack rate is still too low, suggesting that a significant majority of the population would develop measles, which is not accurate for an unvaccinated area.
D. A 90% attack rate accurately represents the situation in areas with low vaccination rates, where a significant portion of the population would develop measles upon exposure to the virus.

**Clinical Pearls:**

1. Immunization plays a crucial role in controlling infectious diseases like measles. By vaccinating a large proportion of a community, herd immunity is achieved, which helps protect both