West Nile Virus
The West Nile virus is a type of virus known as a flavivirus. Researchers believe West Nile virus is spread when a mosquito bites an infected bird and then bites a person.
Mosquitoes carry the highest amounts of virus in the early fall, which is why the rate of the disease increases in late August to early September. The risk of disease decreases as the weather becomes colder and mosquitoes die off.
Although many people are bitten by mosquitoes that carry West Nile virus, most do not know they've been exposed. Few people develop severe disease or even notice any symptoms at all. Those at most risk for developing a more serious form of the disease include people with weakened immune systems, the elderly and very young and pregnant women.
West Nile virus may also be spread through blood transfusions and organ transplants. It is possible for an infected mother to spread the virus to her child through breast milk.