H1N1 severe form of disease, the virus directly infects the lung, causing severe respiratory failure.

“H1N1 infections … clinicians from around the world are reporting a very severe form of disease, also in young and otherwise healthy people, which is rarely seen during seasonal influenza infections. In these patients, the virus directly infects the lung, causing severe respiratory failure.”

http://www.who.int/csr/disease/swineflu/notes/h1n1_second_wave_20090828/en/index.html

in North America