OLATHE, Kan. (AP) — Kansas health officials say two Johnson County residents have developed serious West Nile Virus infections.

Greg Lakin, medical officer of the Kansas Department of Health and Environment, said Tuesday most cases of West Nile are not serious but the two Johnson County residents have neuroinvasive infections, which cause brain swelling and sometimes death.

The health department said most of the state is under a high-risk warning for the mosquito-borne illness.

Health officials say late summer and early fall is the peak time for West Nile Virus. The western two-thirds of the state is the most high-risk part of Kansas, while northeast Kansas is at moderate risk.

There were more than 600 cases of neuroinvasive West Nile Virus and 30 deaths in Kansas from 1999 to 2017.