Pneumonia is an illness of the lungs and respiratory system in which the alveoli (microscopic air-filled sacs of the lung responsible for absorbing oxygen from the atmosphere) become inflamed and flooded with fluid. Pneumonia can result from a variety of causes, including infection with bacteria, viruses, fungi, or parasites. Pneumonia may also occur from chemical or physical injury to the lungs, or indirectly due to another medical illness, such as lung cancer or alcohol abuse. Typical symptoms associated with pneumonia include cough, chest pain, fever, and difficulty breathing. Diagnostic tools include x-rays and examination of the sputum. Treatment depends on the cause of pneumonia; bacterial pneumonia is treated with antibiotics. Pneumonia is a common illness, occurs in all age groups, and is a leading cause of death among the elderly and people who are chronically ill. Vaccines to prevent certain types of pneumonia are available.