Is cat Pneumonia treatable?

My families cat went to the vet and the vets think that she has Pneumonia is it treatable. Well the vet checked her and took test but it didnt come out to be Pneumonia but he said that since one side of her lungs souds well and the others doesnt. And she was laying around- not being her normal self. So if you know whats wrong or if it is Pneumonia please answer back. ASAP