Why Hasn't Penicillin Wiped Out All Pneumonias?

QUESTION: If antibiotics are so effective, they should be capable of
destroying all the germs responsible for disease. Why hasn't penicillin wiped
out all pneumonias?
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ANSWER: Although the penicillin antibiotics are effective against many forms
of pneumonia, it won't treat them all. Before antibiotics were invented,
almost all cases of pneumonia were attributed to streptococcal bacteria
(Streptococcus pneumoniae), which respond well to penicillin. Now, strep
pneumonia accounts for only 55% to 76% of cases. Other causes include
Legionella (which causes Legionnaire's disease) and mycoplasma, a fungus. An
additional problem is that pneumonias that were once susceptible to penicillin
antibiotics are growing increasingly resistant.
To diagnose pneumonia, a physician tests a sample of sputum coughed up
from the lungs searching for the bacterial culprit. Many physicians treat
most pneumonia cases empirically, that is they administer an antibiotic such
as penicillin to kill the most likely causes before they get the test results
back. If the first antibiotic doesn't work, then the test results are used to
help choose a more specific or stronger antibiotic.
Despite research, which continues to discover new and more affective
antibiotics, I doubt that the day will ever arrive when a single medication
will totally eradicate an infectious disease. Prevention is the name of the
game.