**Core Concept**
Chickenpox, caused by the varicella-zoster virus, is a highly contagious illness that spreads through respiratory droplets, contact with infected individuals, and contaminated surfaces. The duration of infectiousness is crucial for public health and patient management.

**Why the Correct Answer is Right**
The infectivity of chickenpox typically lasts from 1-2 days before the onset of the rash to 5-6 days after the appearance of the rash. During this period, individuals are highly contagious and can spread the virus to susceptible individuals. The virus is present in the respiratory secretions and can be transmitted through direct contact with infected individuals or contaminated surfaces.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the infectivity of chickenpox does not last only a few days before the onset of the rash. The virus is present in the body for several days before symptoms appear.
* **Option B:** This option is incorrect because the infectivity of chickenpox does not last for 1 week after the appearance of the rash. The virus is still contagious for 5-6 days after the rash appears, but not for an entire week.
* **Option C:** This option is incorrect because the infectivity of chickenpox does not last for the entire duration of the rash. The virus is contagious for a shorter period, typically 5-6 days after the rash appears.

**Clinical Pearl / High-Yield Fact**
It's essential to note that individuals with weakened immune systems, such as those with HIV/AIDS or undergoing chemotherapy, may remain infectious for a longer period and require prolonged isolation measures.

**Correct Answer: C. 5-6 days after the appearance of the rash.**