About.com explains that there are five main causes for pneumonia and at least one of them is contagious. Pneumonia is an inflammation of lung tissue produced by a viral, bacterial, fungal or mycoplasma infection. While viral and bacterial infections do not cause pneumonia in other people, mycoplasma pneumonias are contagious.
Both WebMD and About.com mention that pneumonia appears more frequently in people with a weak or compromised immune system. Accordingly, cigarette smokers, alcoholics, the elderly, young children and people suffering from other diseases are at a greater risk. Viruses and bacteria may spread and cause respiratory problems even in healthy people, but these don't usually develop into pneumonia. Finally, chemical pneumonia is never contagious.