Article continues below advertisementDiane Keaton's health declined rapidly in the final days before her sad death.New details surrounding the Annie Hall actress' quick decline have been exposed following the reveal of Keaton's official cause of death - primary bacterial pneumonia. According to her death certificate, Keaton died within days after contracting the lung infection.Article continues below advertisementSource: MEGADiane Keaton died at age 79 on Saturday, October 11.