**Core Concept:** Infectivity period of measles refers to the duration during which an infected person can transmit the virus to others before and after the onset of symptoms. Measles is a highly contagious viral disease caused by the measles virus, which belongs to the Paramyxoviridae family.

**Why the Correct Answer is Right:** The correct answer is **D**: 4 days. Measles virus remains infectious in the respiratory secretions of infected individuals for up to four days before the appearance of symptoms and up to 4 days after the onset of rash. This is why it is so contagious and why quarantine measures are often employed to prevent the spread of measles.

**Why Each Wrong Option is Incorrect:**

A. 2 days: This is too short as the virus remains infectious at least 4 days before and after the onset of symptoms.
B. 6 days: This is too long as the virus remains infectious for 4 days after the appearance of symptoms.
C. 8 days: This is too long as the virus remains infectious for 4 days before and after the onset of symptoms.

**Clinical Pearl:** Understanding the infectivity period of measles is crucial for healthcare professionals in controlling the spread of the disease and implementing appropriate infection control measures.

**Correct Answer:** D. 4 days.