A somewhat rare and more deadly version of typical
bacterial
pneumonia, the viral type can kill within two days. Incidence is much lower: 2 out of 10,000 annually for viral vs. 1 in 100 for bacterial. Often fatal in the young,
immunocompromised, and elderly, viral pneumonia can also kill perfectly healthy young people. Several viruses can lead to an infection, including respiratory syncytial virus, or RSV,
influenza,
hantavirus and
Herpes simplex virus.
Viral pneumonia killed Jim Henson.
Sources:
http://medlineplus.adam.com/ency/article/000073.htm
http://www.emedicine.com/emerg/topic468_2.htm