How deadly is pneumonia?

Can be fatal if untr: If left untreated couyld be serious depending on if its viral or bacterial. See your doctor.

Varies: Some forms are mild and self heal over days/weeks. These are often viral. Some forms are very aggressive and deadly. Jim Henson, the puppet master that started the Muppets died from pneumococcal pneumonia in a few days after initially shrugging off symptoms to work. By the time he went in they couldn't stop it from killing him.