Secondary attack rate of chicken pox is

**Core Concept**
Chickenpox, caused by the varicella-zoster virus, is a highly contagious disease that spreads from person to person through respiratory droplets and direct contact. The secondary attack rate is a measure of the risk of infection among individuals exposed to a primary case.

**Why the Correct Answer is Right**
The secondary attack rate of chickenpox is approximately 90%, indicating that about 9 out of 10 susceptible individuals exposed to a primary case will develop the disease. This high rate is due to the virus's ability to spread rapidly through direct contact with infected individuals, particularly in crowded environments like schools and households. The virus can also spread through respiratory droplets, such as those produced when an infected person coughs or sneezes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because the secondary attack rate for chickenpox is generally considered to be high, but the exact percentage may vary depending on factors like vaccination status and population immunity.
**Option B:** This option may be tempting, but the actual secondary attack rate for chickenpox is significantly higher, making it an incorrect choice.
**Option C:** This option is probably incorrect because the secondary attack rate for chickenpox is not typically reported to be as low as this option suggests.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the secondary attack rate for varicella (chickenpox) can be significantly reduced by vaccination, making it a crucial tool in preventing outbreaks and controlling the spread of the disease.

**Correct Answer: C. 70%**