"Our tests show signs of a severe blood infection [sepsis]," said Ze'ev Rotstein, director general of Sheba Medical Center in Tel Hashomer.
Rotstein said Sharon was "fighting like a real warrior, like he did throughout his life," Haaretz reported.
Rotstein said he thought Sharon, 85, did not have long to live but the doctors treating him "remain hopeful."
On Thursday, Rotstein had said Sharon, in a coma for eight years and organs failing, was in critical condition. Officials said his family was maintaining a vigil at the hospital.
Sharon has been suffering from renal failure in recent days. Rotstein said Wednesday Sharon had not undergone dialysis, but was "still getting the same medical treatments he has been getting for years."
Sharon has been comatose since Jan. 4, 2006, when he suffered a brain hemorrhage.