microbiology
Last reviewed 01/2018
The organisms usually implicated are bacteria, including: (1)
- Streptococcus pneumonia
- Staphylococci
- Haemophilus influenzae
-
-hemolytic and 
-hemolytic streptococci - Klebsiella pneumonia
- Escherichia coli
- anaerobes
Less commonly, viruses are implicated, for example:
- rhinovirus
- parainfluenza virus
- influenza virus
Reference: