I know you recently went through it too.  It is always awful.
This was the most bizarre thing.  A seemingly perfectly healthy fairly young (just turned seven) dog suddenly has severe pneumonia.  We think there had to be something else going on, just never figured out what started it.  I suspect he must have had cancer or something somewhere.  Only thing I had noticed is his bark had changed pitch the last few months.  Sure is quiet in this house alone.