Which of the following diseases is NOT transmitted by mosquitoes?

The flu virus is not transmitted by mosquitoes, making it the correct choice. The flu, or influenza, is primarily spread from person to person through respiratory droplets when an infected person coughs or sneezes. It does not rely on mosquito vectors for its transmission.

In contrast, La Crosse encephalitis, West Nile Virus, and Eastern equine encephalitis are all diseases that are known to be transmitted by mosquitoes. These diseases are caused by viruses that utilize mosquitoes as vectors, which means mosquitoes bite infected animals or humans, acquire the virus, and then transmit it when they bite other hosts. Therefore, the identification of the flu virus as the only option that does not involve mosquitoes is accurate and reflects an essential understanding of disease transmission dynamics related to mosquito control.