**Core Concept**
Chemoprophylaxis refers to the use of medications to prevent disease in susceptible individuals, typically in high-risk settings or after exposure. It is most effective when applied to infections with well-defined transmission routes and available preventive agents, especially those where antibiotics or antivirals can interrupt the infectious process.

**Why the Correct Answer is Right**
Typhoid fever is caused by *Salmonella typhi*, a bacterium that is primarily transmitted via the fecal-oral route. Unlike other diseases listed, typhoid is not treated or prevented with chemoprophylaxis in routine settings. Instead, prevention relies on vaccination (e.g., typhoid vaccine) and improved sanitation. Chemoprophylaxis is not routinely recommended for typhoid due to lack of proven efficacy and the risk of antibiotic resistance. In contrast, meningococcal meningitis (B), cholera (C), and plague (D) have established chemoprophylactic regimens (e.g., ciprofloxacin, doxycycline, streptomycin) used post-exposure or in outbreak settings.

**Why Each Wrong Option is Incorrect**
Option B: Meningococcal meningitis – Chemoprophylaxis with rifampin or ciprofloxacin is standard after exposure or in outbreak settings.
Option C: Cholera – Oral rehydration and tetracycline or doxycycline are used for chemoprophylaxis in endemic areas.
Option D: Plague – Streptomycin or doxycycline are used for chemoprophylaxis in exposed individuals.

**Clinical Pearl / High-Yield Fact**
Typhoid is **not** prevented by chemoprophylaxis; instead, **vaccination and hygiene** are the main preventive strategies. Chemoprophylaxis is only recommended for specific, acute exposures to certain bacterial diseases like plague or meningococcal infections.

✓ Correct Answer: A. Typhoid