The Centers for Disease Control says there are nearly 700 cases in 43 states.
Over 80-percent of the cases are in six states: Texas, California, Mississippi, Louisiana, Oklahoma, and South Dakota.
Texas is considered the outbreak's epicenter. Ten people there died from the virus and more than 200 have gotten sick. Experts say hot, dry weather in Texas is exacerbating the outbreak.
26 human cases of the virus that is spread by mosquitoes have been reported in California.