WebMD turned to three experts and asked them to address the West Nile virus questions most commonly asked.
Who is most at risk?
"Looking at the risk of getting infected, anyone who is outdoors and participating in activities is,'' says Erin Staples, MD, PhD, medical epidemiologist at the CDC.
That's especially true at dusk and dawn, she says, when mosquitoes carrying the virus are most active. "Only 1 in 5 people who get infected will develop symptoms," she says.
In most cases, the symptoms are flu-like and fleeting. But not always. Some people develop severe neurological problems.
"Less than 1% of persons who get infected will go on to develop encephalitis or meningitis," she says. These severe neurological symptoms raise the death risk from the virus.
Anyone 50 or older is more at risk for developing the more severe symptoms, she says.
"Clearly the older you are, starting at 50 or so, severity [of the infection] is likely to increase as are death rates," says William Schaffner, MD, an infectious disease specialist and chair of preventive medicine at the Vanderbilt University School of Medicine in Nashville.
Using data reports from cases, the CDC has identified certain medical conditions that raise your risk of becoming infected, Staples says. They include:
High blood pressure
"As far as we know, pregnant women are not necessarily at higher risk of getting infected," she says. "The data we have is somewhat limited. We have not had a lot of pregnant women being infected."
The cases they have looked at have been few, she says. It is difficult to draw any conclusions.
"I know of no data that says if they are infected it will imperil their pregnancy," Schaffner says. Much needs to be learned, he says. "This is a relatively new infection and we should all stay tuned."
Information on infants is limited, Staples says. While there have been periodic reports of infants infected, she could not cite an exact number since the virus was first identified.
"So far this year, we are only aware of one [infected] infant that has been reported," she says.
Infants don't seem to be at increased risk for severe illness, she says, ''but that is based on limited data."
The virus was first identified in the U.S. in 1999.