## Core Concept
The yellow fever vaccine is a live, attenuated vaccine used to protect against yellow fever, a viral disease transmitted by mosquitoes. The vaccine provides immunity against the disease and is especially recommended for travelers to areas where yellow fever is common. The validity period of the vaccine is crucial for international travel requirements.

## Why the Correct Answer is Right
The yellow fever vaccine is valid for 10 years, after which a booster dose is required to maintain immunity and comply with international health regulations. This 10-year validity period is a critical piece of information for travelers and healthcare providers. The vaccine is administered in a single dose, and the booster dose is recommended every 10 years to ensure continued protection.

## Why Each Wrong Option is Incorrect
* **Option A:** 5 years - This is incorrect because the validity period of the yellow fever vaccine is not 5 years; it is actually 10 years.
* **Option B:** 20 years - This is incorrect because there is no evidence to support a 20-year validity period for the yellow fever vaccine; the recommended duration is 10 years.
* **Option D:** Lifetime - This is incorrect because, although some vaccines offer lifetime immunity, the yellow fever vaccine requires a booster dose every 10 years to maintain immunity.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the yellow fever vaccine's 10-year validity period is crucial for travelers to endemic areas, and a booster dose is required every 10 years to ensure continued immunity and compliance with international travel regulations.

**Correct Answer: C. 10 years.**