The driver was going about 83 mph in a 25-mph zone in June when his SUV plowed into the car of Frances Wilk, who was waiting at a red light. Prosecutors investigated for eight months before deciding not to file charges, Daily Herald Media reported.
The 51-year-old won't even be cited, the police department said in a statement Friday.
"No criminal statutes were violated," Wausau Police Capt. Ben Bliven said. "Because of the medical condition, a speeding citation will not be issued."
Police say privacy laws prevent them from releasing the man's name.
Medical records say the man's neurologist believes the man had a seizure while driving, which ultimately caused the crash, police said.
The man has multiple sclerosis and a history of seizures, according to police.
The man lost his license for four months in 2012 after a previous seizure, police said. Officers completed a driver-behavior report after that incident, which resulted in the state Department of Transportation suspending his license. It was reinstated in September 2012.
The man lost his driving privileges again after the June collision. He won't get his license back without a doctor's permission, Bliven said.