Diane Keaton Struggled With Illness for a While Before Pneumonia Death

Diane Keaton's family says she died from pneumonia ... and a longtime friend tells TMZ the actress' death didn't come as a surprise because she had been ill for a while.

A longtime friend of Diane's told us she was struggling with an unnamed illness leading up to her October 11 death ... and the decline in Diane's health meant news of her passing was "not a shock" to the pal.

Diane died this past Saturday at age 79 ... and as TMZ first reported, the L.A. Fire Dept. paramedics responded to her home around 8:08 AM, transporting one person -- confirmed by sources as Keaton -- by ambulance to a nearby hospital.

Dispatch audio featured first responders calling a "person down" at her address.

Diane's family announced Wednesday the cause of death was pneumonia.

It will be interesting to see what's on her official death certificate, because it sounds like there may be some underlying causes and/or contributing factors.