Many different kinds of viruses, bacteria, and fungi can cause the disease. Most commonly, infection occurs through airborne transmission (inhaling infectious particles, produced by a cough or sneeze, that stay in the air for a period of time) or droplet transmission (when infectious droplets produced by coughing, sneezing, eating, or talking come into contact with the eyes, nose, or mouth). Aspiration pneumonia occurs when a person who has difficulty swallowing suddenly inhales food particles, which then irritate the airways and cause infection.