**Core Concept**
Measles is a highly contagious viral disease caused by the measles virus, primarily spread through respiratory droplets. Effective control measures for measles include vaccination and strict isolation of infected individuals.

**Why the Correct Answer is Right**
Measles does not require chemoprophylaxis because it is caused by a virus, not a bacterial infection. Chemoprophylaxis is the administration of medication to prevent disease, typically effective against bacterial or parasitic infections. Since measles is a viral disease, antiviral medications are not effective in preventing its spread or infection. However, vaccination is a highly effective method of preventing measles, and strict isolation of infected individuals can also help control outbreaks.

**Why Each Wrong Option is Incorrect**
**Option B:** Conjunctivitis, an infection of the conjunctiva, can be caused by various pathogens, including bacteria, viruses, and allergens. Chemoprophylaxis may be required in cases of bacterial conjunctivitis to prevent the spread of infection.

**Option C:** Cholera, caused by the bacterium Vibrio cholerae, is a waterborne disease that can be prevented through chemoprophylaxis with antibiotics in high-risk areas or in individuals exposed to the pathogen.

**Option D:** Malaria, caused by Plasmodium parasites transmitted through mosquito bites, can be prevented through chemoprophylaxis with antimalarial medications in high-risk areas or in individuals traveling to endemic regions.

**Clinical Pearl / High-Yield Fact**
When considering chemoprophylaxis, it's essential to remember that not all infectious diseases require medication to prevent their spread. Vaccination and strict isolation measures can be effective alternatives, especially for viral diseases like measles.

**✓ Correct Answer: A. Measles**