Fevers, weight loss, cough productive of the worst smelling sputum on the planet. CXR has a usually thick walled cavity with an air fluid level, often it erodes into pleural space to cause an empyema.
If you have never smelled an anerobic pneumonia, mix a pound of hamburger with some stool and spit and put it in a sealed bag in a warm place for a week. Open, sniff, then barf.
Poor dentition and loss of consciousness (from trauma, ETOH, drugs or seizures) with resultant aspiration.
Lung abscess is the real, and only, aspiration pneumonia where anaerobes are a pathogen. In all aspiration events, whether in the community, the hospital or the ventilated patients, anaerobes are never, ever (that is a never and an ever) an issue and never ever need to be treated.
Many other organisms can cause cavitary pneumonia including, but not limited to Actinomyces, Aspergillus, Blastomyces, Coccidioides, Corynebacterium pseudodiphtheriticum, Cryptococcus, Echinococcus, Entamoeba histolytica, Histoplasma, Legionella, Mycobacteria, Nocardia, Paragonimus westermani, Pneumocystis (esp on inhaled pentamidine), Pseudomonas aeruginosa, Staphylococcus aureus, Zygomycetes.
Penicillin G or ampicillin PLUS metronidazole (my favorite); a third generation cephalosporin could substitute for the ampicillin) OR penicillin/beta-lactamase inhibitors OR clindamycin OR cefoxitin OR cefotetan OR a quinolone PLUS metronidazole.
For a classic lung abscess my go to inhouse is Ceftriaxone and oral metronidazole. Outhouse? If nothing odd or resistant oral amoxicillin and metronidazole. If I don't like the patient I suggest Augmentin: costs a lot more and will almost certainly cause diarrhea.
For treatment of other causes, see the specific organism.
If antibiotics fail, percutanous drainage can help with little in the way of morbidity (PubMed).
Given the increased resistance of streptococci and anaerobes to clindamycin (both around 30%), I no longer use it. Once therapy is begun, expect fevers for at least 10 - 14 days.
Hospital, especially ICU, aspirations DO NOT INVOLVE ANAEROBES.
Relevant links to my Medscape blog