**Question:** All of the following are true about measles except?

A. Measles is a highly contagious viral disease
B. Measles primarily affects children
C. Measles is a vaccine-preventable disease
D. Measles is caused by a bacterium (not a virus)

**Core Concept:** Measles is a highly contagious viral disease that primarily affects children. It is caused by the measles virus (family Paramyxoviridae, genus Morbillivirus) and is a vaccine-preventable disease. The virus enters the body through respiratory or digestive tract mucous membranes or breaks in the skin and spreads through respiratory droplets.

**Why the Correct Answer is Right:** Option D is wrong because measles is caused by a virus, specifically the measles virus. The virus belongs to the family Paramyxoviridae, genus Morbillivirus. The virus enters the body through respiratory or digestive tract mucous membranes or breaks in the skin and spreads through respiratory droplets.

**Why Each Wrong Option is Incorrect:**

A. While measles is a highly contagious disease, Option A is correct because it highlights the contagious nature of the disease. Measles is one of the most contagious viral diseases, with a reproductive number (R0) of around 12-18, meaning each infected person can infect 12-18 others.

B. Measles primarily affects children, Option B is correct because measles is more common in children due to their lower immunity and lack of immunity acquired from mothers during pregnancy or breastfeeding. However, measles can affect people of all ages.

C. Measles is a vaccine-preventable disease, Option C is correct because vaccines are available to prevent measles. The Measles, Mumps, and Rubella (MMR) vaccine is commonly used to prevent measles, mumps, and rubella. The vaccine consists of a combination of live attenuated viruses (specifically, the measles, mumps, and rubella viruses) that stimulate the immune system to produce immunity without causing the actual disease.

**Clinical Pearl:** The measles vaccine is highly effective, with over 95% seroconversion rate, meaning that at least 95% of vaccinated individuals develop immunity to measles after vaccination. However, in areas with lower vaccination rates, outbreaks of measles still occur, leading to high morbidity and mortality rates, especially in immunocompromised individuals and unvaccinated individuals. Measles can lead to severe complications, such as pneumonia, encephalitis, and diarrhoea, which are responsible for most of the mortality.