**Core Concept**
Measles is a highly contagious viral infection caused by the measles virus, which primarily affects the respiratory system. The virus is transmitted through respiratory droplets and remains active in the environment for a short period.

**Why the Correct Answer is Right**
The reservoir for measles is humans, specifically those who are infected with the virus or have been vaccinated. The measles virus can remain latent in the respiratory tract for a short period, allowing it to be transmitted to susceptible individuals. The virus replicates in the respiratory epithelial cells and is shed in respiratory droplets, leading to further transmission.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because measles is not typically associated with animals as a reservoir.
**Option B:** This option is incorrect because the measles virus is not commonly found in water or food sources.
**Option C:** This option is incorrect because while the measles virus can be transmitted through contact with an infected person, the primary reservoir is not the human gut.

**Clinical Pearl / High-Yield Fact**
Measles is a highly contagious disease that can be prevented through vaccination. The measles vaccine is typically administered in two doses, with the first dose given at 12-15 months of age and a booster dose given at 4-6 years of age.

**Correct Answer: D. Humans.**