**Core Concept**
Measles is a highly infectious viral disease primarily affecting young children, characterized by fever, cough, runny nose, and a characteristic rash. The secondary attack rate in measles is significantly higher due to the high contagious nature of the virus.

**Why the Correct Answer is Right**
The secondary attack rate in measles is actually around 90-95% among susceptible contacts, not 30%. This is because measles virus is highly contagious and spreads through respiratory droplets, direct contact with infected individuals, and contaminated surfaces. The high secondary attack rate is a key factor in the spread of measles outbreaks.

**Why Each Wrong Option is Incorrect**
* **Option A:** Maximum incidence in 6m - 3 year age group is true. Measles typically affects young children, and the highest incidence is observed in this age group.
* **Option B:** Best age for immunization is 9-12 months is true. The measles vaccine is generally administered at 9-12 months of age, with a second dose at 16-18 months. This timing helps protect children from measles before they reach the age of highest susceptibility.
* **Option C:** Secondary attack rate is 30% is false, as explained above. A secondary attack rate of 30% is not characteristic of measles.

**Clinical Pearl / High-Yield Fact**
The measles vaccine is highly effective, with two doses providing around 97% protection against measles. However, a single dose provides around 93% protection, highlighting the importance of completing the vaccination series.

**✓ Correct Answer: C. Secondary attack rate is 30%**