THERE ARE TWO main types of West Nile virus capable of causing disease. Scientists have identified these two major types, or lineages, of the virus by using sophisticated gene sequencing techniques.

These methods involve chemically separating the virus's DNA or RNA strands, the structures which house genes. Once these strands are separated, the genes on them can be mapped and sequenced with the aid of a computer. This identifies where different genes are on the DNA or RNA molecules. Different gene sequences characterize different lineages of a particular virus. Investigators have discovered that only one of the two known lineages of West Nile virus causes human West Nile fever and West Nile encephalitis, meningitis, and meningoencephalitis. This group is known as Lineage One. Lineage One virus has been conclusively identified as causing West Nile virus outbreaks...