West Nile virus is spread through the bite of an infected mosquito. The mosquito can become infected after biting an infected bird and this can then be transmitted from the bird to humans.
There is no evidence to suggest that West Nile virus can be spread from person to person or animal to person.
The West Nile virus was first discovered in the West Nile area of Uganda in 1937.
It is prevalent in North America, where two thirds of the East coast states have had cases. There has even been a case in California.
There have also been cases in Israel, Africa and Asia and parts of Eastern Europe.
There was a UK case of a bird with the antibodies of the West Nile virus, but there have been no human cases.
The patient can incubate the disease for up to a fortnight after being bitten.
Many of the people who become infected will have either no symptoms or mild flu-like symptoms, which last a couple of days.
These can range from fever, headache and body aches to skin rashes on the trunk of the body and swollen lymph glands.
But more serious cases, less than 1% of those who become infected, can result in severe illness such as West Nile encephalitis or meningitis and sometimes lead to death.
Out of about 1,000 people who became infected in Europe recently there were about 40 deaths.
Scientists have developed a vaccine for the disease and it was due to start human trials in late 2002.
But as yet there is no vaccine for travellers to use so the best way to protect yourself is to avoid being bitten by a mosquito.
The mosquitoes which carry this disease bite at dawn, dusk and early evening, so if you are travelling to an area where the disease is prevalent you should either stay indoors, or wear long sleeved shirts and long-trousers and wear insect repellent.
People travelling with young children should ensure that they use a specially prepared insect repellent, rather than the adult version.