West Nile Virus was first reported in the U.S. in 1999. The virus spreads through mosquito bites, although most people who catch it won’t experience symptoms. About 20% of those infected will develop symptoms such as fever. Less than 1% of those infected will develop a more severe and sometimes fatal illness.

Weld County had 15 reported cases of human West Nile virus, according to the website. The year before reached 52 cases.