"Less than 1% develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues), " it said.

CNN: West Nile virus on the rise in the U.S., health officials say

"Less than 1% develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues), " the CDC said.

CNN: SHARE THIS

However, the vaccine can have serious side effects -- including encephalitis, which can sometimes be fatal -- so health officials want to give it only to people who need it.

CNN: Administration to buy smallpox vaccine