Anyone can get pneumonia, but it’s most common in people older than 65, infants and very young children, and people with weakened immune systems (for example, due to a medical condition or the use of immunosuppressive medications).
Pneumonia might affect one lung, in which case it’s called unilateral pneumonia, or it might strike both lungs, called bilateral pneumonia. All of this makes it harder for oxygen to get into your bloodstream and leads to difficulty breathing. When your lungs are infected, however, the alveoli fill up with fluid or pus rather than air, while the bronchioles may become inflamed and swollen, making it harder for air to move through them. The alveoli are covered in very small blood vessels called capillaries, which get oxygen from the alveoli to carry to the cells throughout your body and return carbon dioxide to be exhaled. The bronchioles culminate in millions of tiny air sacs called alveoli. These bronchi branch off into smaller bronchi, which in turn branch off into thousands of even smaller airways called bronchioles. Your trachea, or windpipe, leads into an airway, or bronchus (plural: bronchi) that connects with each lung.
To help visualize this, let’s quickly review the anatomy of your lungs. Specifically, pneumonia affects your lungs’ air sacs (alveoli) and the tissue surrounding them. Pneumonia is an infection in your lungs, usually caused by viruses or bacteria less commonly, it can be caused by a fungus.