(HealthDay News) -- Pneumonia is a lung infection caused by a bacterium, virus or fungus. Often, the lungs' air sacs fill with fluid or pus, making it difficult to breathe.

The National Heart Lung and Blood Institute says risk factors for pneumonia include:

• Being 65 or older, or a baby aged 2 years or younger. Both age groups tend to have weaker immune systems.
• Having a lung disease or condition that affects breathing, including cystic fibrosis, chronic obstructive pulmonary disease or asthma.
• Having a chronic disease such as heart failure, diabetes or sickle cell anemia.
• Being in a hospital's intensive care unit, particularly if you need a ventilator to breathe.
• Having certain complications of stroke, including coughing or problems swallowing.
• Being malnourished.
• Smoking or drinking too much alcohol.