What is pneumonia, the lung infection that caused Diane Keaton's death?

Beloved Oscar-winning actor Diane Keaton died from pneumonia, according to a statement her family shared with People magazine.

Beloved Oscar-winning actor Diane Keaton died from pneumonia, according to a statement her family shared with People magazine.

It was not clear if she had any underlying health conditions that contributed to her death on Saturday at 79.

Here's everything you need to know about the lung infection, what causes it and how it led to Keaton's death.

Pneumonia is an infection that inflames the sacs in one or both lungs, according to the Mayo Clinic.

Those sacs of air fill with fluid or pus, causing a cough with phlegm or pus, fever, chills and trouble breathing, according to the clinic. The illness can range in seriousness from mild to life-threatening, and it can cause death in severe cases.