Technically, “pneumonia” means “lung infection,” which can be caused by any germ. But in common language, we mean bacteria (see Differences Among Germs). The most common bacteria causing pneumonia is Streptococcus pneumoniae (“pneumococcus”), which we get from germs living harmlessly in our nose & throat. Before antibiotics were invented, over a third of patients died (over 80% if elderly).
Other bacteria are lots milder, like Mycoplasma pneumoniae (sometimes called “walking pneumonia,” because patients aren’t bedridden). Some are worse, like Staph. Mycoplasma is contagious; Legionnaires’ lives in water coolers & occurs in outbreaks; some pneumonias only occur in AIDS (Pneumocystis). You get Psittacosis pneumonia from parrots. Etc.
We think of pneumonia when a patient has cough with a fever, and it’s not influenza season, and a test for Covid-19 is negative. Many patients cough up phlegm that’s green, rusty, or blood-tinged. Some patients have chest pain with every breath, and maybe no cough at first. Pneumonia in the lower right lung can cause pain in the upper right abdomen, which can be confusing.
Chest x-ray makes the diagnosis, but we don’t order one if the patient doesn’t look sick and isn’t short of breath, since they probably just have a simple virus. We do order an x-ray if we hear crackles in the lungs (by stethoscope), if there’s chest pain with every breath, if symptoms haven’t improved by the third day, or if they’re elderly or have a chronic illness. In children, if we hear typical crackles, we diagnose & treat pneumonia without the x-ray.
It can be hard, or impossible, to identify the exact germ. But there are standard ways of deciding how to treat, and whether or not a patient should be hospitalized. With antibiotic treatment, over 99% of patients survive. But among patients who are sick enough to need hospitalization, mortality may be 13%, and 36% if they need an intensive care unit (even with antibiotics!).
There are 2 vaccines to prevent pneumonia, which aren’t perfect. We give them to children <2 years-old, adults >65, and anyone with certain chronic illnesses or who smokes.