Question

In 1943, drug companies began mass-producing penicillin. At that time, it was a medical miracle and was used as a remedy for infected wounds, pneumonia, and other bacterial diseases. Just four years later, however, it was no longer effective on the same diseases that it had previously been able to cure. What happened?

Answer 1

Some of the bacteria mutated to become resistant to penicillin.

Answer 2

The bacteria that caused diseases treated by penicillin became resistant to the drug. The overuse of penicillin easily narrowed the populations of disease-causing bacteria to the few strains that were resistant to penicillin, and these few strains grew to populate. Natural selection.