West Nile virus is a viral disease that is transmitted by mosquitoes. The virus is a member of the Flaviviruses, which also includes Zika, dengue and yellow fever viruses. The virus was first identified in the West Nile district of Uganda in 1937,

Philadelphia health officials are warning residents after West Nile virus was found in mosquitoes in Northeast Philadelphia earlier this month.

Mosquitoes in a Butler County community have tested positive for West Nile virus. The Mercer County Conservation District says the mosquitoes collected in Franklin Township were the first sample to test positive for the virus in Butler County in 2025.