Pneumonia is an infection caused by both bacteria and virus. It attacks the air sacs in one of both lungs. The infected sac(s) may fill with phlegm and cause cough and difficulty in breathing.
With winter starts flu season when people start getting hit by influenza and norovirus a.k.a. winter vomiting virus or stomach flu. But doctors often focus more on influenza partly because vaccines are available to prevent this virus.