alpha strep pneumonia-why is it so severe?

We have had several several apparently healthy, young adults (late 20s to early 40s) with no previous medical history end up on a vent for days after coming in with pneumonia. I've never seen anything like this...I've seen many elderly pts. or pts with co-morbidityies etc. end up intubated but yeah, I haven't seen this. The mic reports mention several kinds of bugs but I believe the main culprit is alpha streptococcus. I looked on the internet for more information but couldn't find any. What is the patho behind pneumonias that become so severe? Why would a seemingly healthy adult contract them? Very curious. Thanks for your thoughts.
I have heard of a few cases locally-am curious to see others input..

H1N1 flu related?

Group A Strep in pneumonia can also cause empyema along with a host of other problems to include endocarditis, necrotizing faciitis, strep toxic shock syndrome to name a few. Toxic shock can lead to ARDS, renal failure and death. Group A strep can be aggressive with extensive tissue destruction and has a high incidence of morbidity and mortality. Can be a pretty nasty bug.