Prosecutors from the Illinois attorney general's office told Judge James Stewart Monday that the unidentified woman has cancer and has been given weeks to live.
The woman will testify Wednesday in a closed-door hearing, and her testimony will be sealed.
It was not immediately clear what information the woman would offer about 29-year-old Nicholas Sheley.
Sheley has pleaded not guilty to murder in the June 2008 death of a 65-year-old Galesburg man. No trial date has been set.
He is also charged in the deaths of five other people in Illinois and two in Missouri, including Tom and Jill Estes of Sherwood.
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