We completed a thorough review of the CXR interpretation in pneumonia and highlighted the following key points for ED practice:

ED Physicians will usually not know the cause of the pneumonia during acute assessment and subsequent admission.

Epidemiological risk stratification is therefore vital to guide empiric antibiotic choice.
Imaging, CT, lab tests usually will not identify the type of organism causing the illness.
Severity assessment is important to predict need for ICU care and guide initial abs choice.

CAP in immunocompetent patients with no other significant comorbidities is likely to be caused by viruses including rev and influenza. The key bacterial causes include
strep / haemophilus / moraxella / mycoplasma / chlamydia trachomatis

alcoholic / diabetic patients – add klebsiella
immunodeficient / hiv patients – staph / p jerovecii / tb
cohorts near water sources – legionella
AIDS or cell mediated immunity deficient – viral pneumonia herpes and cmv

Severity assessment:
PSI / smart cop / curb 65 – know the components of scoring and what the scores identify

A good review of subject in Rosens and also UpToDate.