[MDriver1981]

Why was it so common for people to die of pneumonia before 1945?  My great-grandmother died of pneumonia at age 47 in 1942.  Penicillin was invented in 1928 and yet it wasn't until after World War II that pneumonia began to decrease in severity.

[rex789]

because one can't breathe under water. pneumonia is like drowning ur lungs in water. many points here.

all has to do with physics.
the more the diffusion distance less you will get into your blood. pneumonia increases the distance it requires one O2 molecule to diffuse thru into the blood.
pneumonia causes lung inflamation which again decreases the capillary cells ability to allow diffusion.
lung gets destroyed, you can actually get cavitations from the immune response.
lastly pneumonia can be cause by many bugs, including viruses (atypical) and penicillin does not kill all of them. lastly the elderly (back then 47 would be somewhat elderly)already have a decreased immune response and if you get pneumonia in that condition you are opening the door to a whole bunch of other bugs to colonize the lungs. even today one of the worst things your patient can get is pneumonia, specially a hospital acquired, cuz then you are messing with resistand bugs that don't like to give up. Add to this various other immunocompromising diseases, people with transplants, leukemias, cancers and other immuno deficiencies and you have a formulla for a quick exit from EARTH.