And although most people who contract it show no ill effects, West Nile is capable of doing severe damage to a vulnerable person's brain and spine, and has taken more than 500 lives, explains The Early Show medical correspondent Dr. Emily Senay.
West Nile is, of course, spread by mosquitoes.
Senay stresses that, despite the "alarming" number of cases over the past couple of years, the chances of getting sick from a single mosquito bite are extremely low.
The federal Centers for Disease Control and Prevention estimates that 80 percent of those who become infected with the virus never even know they have it. They never develop any symptoms.
About 20 percent will develop what's called West Nile Fever with flu-like symptoms such as a headache, fever, and muscle aches.
The only way to really know for sure that it's West Nile is by doing a blood test.
Fewer than one percent of those infected will develop a very serious version of the virus called West Nile Neuro Invasive Disease, which can cause inflammation of the brain and spinal cord, and can be life threatening.