West Nile Virus is an infectious disease originally discovered in Uganda. It spread from this area to southern and western Asia, as well as eastern Europe. The virus reached the United States by the late 1990s and has become a danger all over the US. It is transmitted by mosquitoes and is a relative of yellow fever. Like similar flaviviruses, West Nile can produce nausea, vomiting, fever, head and body aches, swollen lymph nodes and rashes, with some people developing extreme weakness and an extremely high fever. 

In rare cases, West Nile Virus can lead to death, with fatality rates of about 3 to 15 percent for patients with the most serious form of the disease. The risk is highest for elderly people and patients who already have compromised immune systems. Anyone with symptoms suggesting West Nile Virus should be tested for it, however, as the disease can worsen unexpectedly. 

The test for West Nile Virus relies on a polymerase chain reaction, or PCR, which is often used with antibody-detection tests such as IgM and IgG. This test is specific for West Nile Virus Lineage 1, from New York in the late 1990s, and does not detect Lineage 2, from Uganda, but the second lineage has not yet been encountered in North America and is considered much less dangerous. PCR West Nile Virus tests are less sensitive than antibody tests, but they are much more specific and don't result in many false positives.