**Core Concept**
Yellow fever vaccination is a live, attenuated vaccine that provides protection against yellow fever virus. The vaccine's efficacy is based on the immune system's ability to recognize and respond to the weakened virus, leading to the development of antibodies and immune cell-mediated responses.

**Why the Correct Answer is Right**
The yellow fever vaccine starts to provide protection 10 days after injection. This is because it takes approximately 10 days for the immune system to recognize and respond to the weakened virus, leading to the development of antibodies and immune cell-mediated responses. The vaccine-induced immune response is essential for neutralizing the yellow fever virus and preventing severe disease. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) recommend that individuals wait at least 10 days after receiving the yellow fever vaccine before traveling to areas where the virus is endemic.

**Why Each Wrong Option is Incorrect**
**Option A:** 5 days - This is too soon for the vaccine to induce a protective immune response, as it takes approximately 10 days for the immune system to recognize and respond to the weakened virus.
**Option C:** 15 days - This is too long, as the vaccine is effective in providing protection within 10 days of injection.
**Option D:** 20 days - This is also too long, and the vaccine's efficacy is not significantly increased beyond 10 days.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the yellow fever vaccine is a live, attenuated vaccine, and individuals with weakened immune systems, such as those with HIV/AIDS or taking immunosuppressive therapy, should not receive the vaccine.

**✓ Correct Answer: B. 10 days**