## **Core Concept**
The secondary attack rate is a measure used in epidemiology to describe the spread of a disease within a defined group, such as a family or a closed community. It is calculated as the number of new cases among contacts divided by the number of exposed contacts. Chicken pox, caused by the varicella-zoster virus, is highly contagious and primarily affects children.

## **Why the Correct Answer is Right**
The correct answer, **90%**, reflects the high contagiousness of chicken pox. This disease spreads through respiratory droplets, direct contact with rash lesions, and possibly through contaminated surfaces. Individuals are contagious from 1 to 2 days before the onset of the rash until all lesions have crusted over, usually around 5-7 days after the onset of the rash. The high secondary attack rate indicates that a large proportion of susceptible contacts will develop the disease if exposed to an index case.

## **Why Each Wrong Option is Incorrect**
- **Option A (10%):** This rate is too low for chicken pox, which is known for its high contagiousness.
- **Option B (50%):** While this seems like a plausible rate for some infectious diseases, it underestimates the contagiousness of chicken pox.
- **Option D (70%):** Although close, this option still underestimates the secondary attack rate of chicken pox.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the secondary attack rate for chicken pox is among the highest for common childhood illnesses, making it a very contagious disease. This high rate underscores the importance of isolation and vaccination strategies to control its spread.

## **Correct Answer:** . **90%**