**Core Concept**
Measles is a highly contagious viral infection caused by the measles virus (MV), belonging to the paramyxovirus family. It primarily affects the respiratory system and is characterized by a distinctive rash, fever, and cough.

**Why the Correct Answer is Right**
The measles virus infects the respiratory epithelium, where it replicates and causes an immune response. The virus is highly contagious, with an incubation period of around 10-14 days. It is spread through respiratory droplets, direct contact with infected individuals, or contaminated surfaces. The measles virus targets the T cells, particularly CD4+ and CD8+ T cells, leading to immunosuppression and increased susceptibility to secondary infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because measles is not primarily a gastrointestinal infection.
**Option B:** Incorrect because measles does not typically cause a severe headache.
**Option C:** Incorrect because measles does not primarily affect the central nervous system.

**Clinical Pearl / High-Yield Fact**
Measles is a vaccine-preventable disease, and the measles vaccine is highly effective in preventing the disease. However, vaccination does not provide lifelong immunity, and booster doses are required to maintain immunity.

**Correct Answer:** Not provided. Please provide the correct answer options for a complete explanation.