The case involves a Glen Ellyn resident in their 50s who became sick earlier this month, health officials said.
Mosquitoes in DuPage County recently tested positive for the West Nile Virus.
Typically, only one in five people infected with the mosquito-borne illness actually feels sick.
Some symptoms include fever, headache, body aches, vomiting or a rash.
Officials said people should wear insect repellent, remove sitting water on your property, and avoid being outside during dusk and dawn when mosquitoes are most active.