Walking pneumonia is changing.
The classic bugs are four "atypical bacteria":
mycoplasma pneumonia
chlamydia pneumonia (this is not the STD chlamydia. Different one.)
legionella
pertussis (whooping cough)
However, streptococcus pneumonia can also be a walking pneumonia OR a lobar pneumonia. In a lobar pneumonia the person usually is short of breath, running a fever of 102-104, and they point to where it is: hurts in the right upper chest. On chest x-ray there will be consolidation: whited out from fluid or swelling instead of nice ribs and dark air. They are often tachycardic and hypoxic.
In walking pneumonia the person often has no or minimal fever, they just feel tired or short of breath when they do things, and the chest xray can be "clear". It isn't really "normal", it's just that the bacteria or virus affects the entire lungs and causes some swelling throughout and doesn't white it out.
"Double" pneumonia is when the chest film is whiting out on both sides. We also see the lungs whiting out with ARDS -- acute respiratory distress syndrome. So after trauma in a car wreck and lots of broken ribs, the lungs can be bruised too and white out. Ow. Influenza virus can cause lung swelling and in the 1917-1918 flu infected military recruits lungs were swelling shut. They would turn blue and die.
"My" strep that I've been writing about is streptococcus A, not strep pneumonia. It causes strep throat mostly though it can invade and cause sepsis or pneumonia or cellulitis. There are currently 4000+ known strains of strep A, and some are resistant to antibiotics or can cause kidney damage or do all sorts of nasty things. I think that "my" strep is resistant to azithromycin.
The current guidelines say to treat walking pneumonia with azithromycin. However, a paper came out this year saying that resistance to azithromycin is rising among streptococcus pneumonia and that nearly 50% of strains tested were resistant. Uh-oh. That means that azithomycin doesn't work and the person can get sicker and may die. I talked to a pulmonologist in Seattle when I needed help with someone. He said that he would have said there weren't any resistant strep pneumo strains here in Washington except that he had one intubated and in the ICU right then. "I'm convinced now, " he said.
A lobar pneumonia is easier to diagnose. Abnormal chest x-ray, reasonably healthy people run a high white blood cell count (so my frail folks, immunosupressed folks and 90 year olds don't), and a fever (ditto) and look sick. The walking pneumonia people come in saying they have been coughing for 3 weeks or 4 weeks or two months. I am doing more lab testing because of the resistance.
This winter I have seen 6 different causes of walking pneumonia here: influenza A, respiratory syncytial virus (In more than one person over 60. That is NOT who the books say it should affect. It's supposed to mostly cause bronchiolitis in babies and preemies), pertussis, strep pneumococcus, strep A and none of the above. All looking pretty much the same, but with different treatment.
https://wwwnc.cdc.gov/eid/article/15/8/08-1187_article
https://www.cdc.gov/pneumonia/atypical/mycoplasma/index.html
http://www.medscape.com/viewarticle/820736
https://www.cdc.gov/drugresistance/threat-report-2013/pdf/ar-threats-2013-508.pdf#page=79
RSV: http://kidshealth.org/en/parents/rsv.html
Mycoplasma resistance to azithromycin has been reported too: http://erj.ersjournals.com/content/36/4/969