Some 86 species of birds have been shown capable of harboring the virus. The birds associated with man, such as the House Sparrow, can be prime reservoirs of this disease. The virus is transmitted to man and other animals by certain species of mosquitoes that feed on infected birds, then bite man and other animals. Morbidity and mortality are greatest in young children and older age groups. The virus is not fatal to the wild birds.
Mosquitoes feed on infected birds and transmit the virus to other birds.
This cycle results in amplification of the virus in the bird populations. The greatest danger occurs in the warm summer months when the virus and the mosquitoes are active. Even then, however, birds that carry the virus are only infective for a period of 4 days. As the amplification cycle continues, and the mosquito population increases, infected mosquitoes bite man and other animals. The unique geological history of this area produced the sandy soils with depressions known as kettles that characterize this region.