**Core Concept:** Western Equine Encephalitis (WEE) is a viral infection caused by the WEE virus, which belongs to the Flaviviridae family. The virus can infect humans and is primarily transmitted through the bite of an infected mosquito, particularly the Culex species mosquito.

**Why the Correct Answer is Right:** The correct answer, option D (mosquitoes), is the primary mode of transmission for the Western Equine Encephalitis virus. The virus is primarily transmitted through the bite of an infected mosquito, particularly Culex species mosquitoes, which are common in many parts of the world, particularly in regions with tropical or subtropical climates.

**Why Each Wrong Option is Incorrect:**

A. Birds (Option A) are not the primary mode of transmission but can act as a reservoir for the virus, meaning they can carry the virus without showing symptoms but can infect mosquitoes.

B. Horses (Option B) are also not involved in the primary transmission, although they can be infected and serve as a sentinel species, indicating the presence of the virus in an area.

C. Humans (Option C) can be infected and develop symptoms, but they are not the primary mode of transmission. Transmission occurs through the bite of an infected mosquito.

**Clinical Pearl:** Understanding the transmission of WEE virus is crucial for preventing and controlling outbreaks. In endemic regions, protecting yourself from mosquito bites is essential, especially during daylight hours when mosquitoes are most active. Using insect repellents, wearing appropriate clothing, and installing mosquito nets or screens can help prevent mosquito bites and subsequent WEE virus infection.