How did West Nile virus most likely enter the U.S. in 1999?

The most likely way West Nile virus entered the U.S. in 1999 is through infected mosquitoes and birds. This is supported by the understanding that West Nile virus is primarily transmitted to humans through mosquito bites, and birds are the main reservoir hosts for the virus. Infected mosquitoes feed on birds, allowing the virus to spread within the avian population and then to humans as the mosquitoes bite other animals or people.

The outbreak in 1999 was traced back to an introduction point where infected birds were found, highlighting the role of avian and vector populations in the virus's transmission cycle. This transmission dynamics emphasizes the ecological relationships between mosquitoes, birds, and humans in the spread of vector-borne diseases, making this option the most plausible explanation for the virus's entry into the U.S. at that time.