Pneumonia was an inflammation of the lungs caused by an infection. Lobar pneumonia was an infection limited to one lobe of a lung, usually caused by Streptococcus pneumoniae.
Elias Sandoval had lobar pneumonia as a child, resulting in scarring on his lungs. This scarring was later eradicated through influence of the Omicron spores. (TOS: "This Side of Paradise")
Following an outbreak of choriocytosis aboard the USS Enterprise in 2269 was finally put under control by Doctor Leonard McCoy, he deemed the disease outbreak as being "no longer even as serious as pneumonia." (TAS: "The Pirates of Orion")