posted 6 years ago
Community acquired is to distinguish from those infections patients pick up after being hospitalized. It's a different set of pathogens, those out in the wide world versus those inside the hospital. (There are some nasty bacteria that hang out in hospitals. If you go to the hospital, wash your hands before you go, for the sake of the sick people in there, and wash again before you leave, for the sake of yourself and your community.)
Doctors use the term to classify illnesses into categories that make it more likely you'll pick the correct antibiotic without actually getting goo out of your patient's lung (in the case of pneumonia) and culturing that to find out exactly what is causing the infection.