Prior to 1999, West Nile fever didnt occur in North America. Now, it returns each year, causing a patchy epidemic with victims across the continent. To understand why this is happening, we have to understand how the virus cycles through birds and mosquitoes in our environment.

Birds carry West Nile virus. It was almost certainly a bird that brought the virus to New York in the summer of 1999, but no one knows for sure exactly how it happened. The bird may have been ill, or it may have been relatively healthy: some birds die from the infection while others are unaffected. In any case the bird was bitten by a mosquito while the virus was circulating in its bloodstream.

The mosquito went on to bite another bird and spread West Nile virus to that bird. More mosquitoes drew blood from both birds, acquiring the virus and subsequently spreading it to more birds. Susceptible birds began to die. Others remained healthy and went on passing West Nile; mosquito infections increased. Somewhere along the line, infected mosquitoes bit people, passing on the virus and eventually causing the first human cases of West Nile fever.

There is no single West Nile mosquito, though some mosquito species may be the principle vector in particular localities. Obviously, mosquitoes that like to feed on birds are likely to be involved in transmitting the agent of West Nile fever. These mosquitoes are also responsible for the reappearance of the virus year after year though West Nile virus may disappear from birds bloodstream over the winter, infected mosquitoes reappear in the spring with last years infection ready to go. Soon the mosquito, West Nile, bird, mosquito cycle starts up again.

Because a couple of species of birds are particularly good at passing West Nile virus to mosquitoes, while remaining healthy themselves, the level of viral infection becomes amplified over the summer months, with more and more mosquitoes infected. At the end of the season, these birds have finished nesting and they tend to disperse, moving out of areas where they have remained for most of the summer. Now the mosquitoes go looking for something else to eat, and they often find humans. This is why the number of cases of West Nile Fever peak in the late summer and early fall.

Then all those infected mosquitoes go into hibernation until next year