West Nile virus is found in tropical and temperate regions and it commonly infects birds, but it can also infect humans, dogs, bats, horses and rabbits. Humans are mainly infected with the virus through the bite of an infected mosquito. The genetic material of the West Nile Virus is a single strand of RNA which has between 11,000 and 12,000 nucleotides long.

The virus has three effects on humans and they are all different. The first one is an asymptomatic infection; the second is a mild febrile syndrome and the third is a neuroinvasive disease, also called West Nile meningitis or encephalitis. West Nile virus is transmitted through mosquito vectors, which bite and infect birds. These birds are only hosts of the virus.

Still, the mammals bitten by the infected mosquitoes don’t get sick so easily because they don’t ingest enough virus to get infected.

This year, in California, the first dead case because of the West Nile virus has been recorded. A 72-year-old woman is the first person who died because of the virus, as the California State Department of Public Health announced on Monday. The woman was from Orange County and her death was caused by complications of the fatal mosquito bite.

98 other people in California got infected with the West Nile virus and suffered serious illnesses because of it. 38 of the 58 California’s counties are under the threat of the killing mosquitoes. 1,295 dead birds have been found in the state until now, together with 1,101 mosquitoes, 5 horses and 109 chickens.