Penicillins were the first identified and widely-used antibiotics, and provided cures and relief from many diseases. The functionality of penicillin is to kill harmful bacteria that cause infection and other problems. The most important positive effect of penicillin is that it increased health and lifespans worldwide, and since it is relatively easy and cheap to produce, it became possible to help people in far-off places. With the advent of penicillin, diseases with a high fatality rate became manageable and even curable; this affected both adult and infant medicine, allowing many more babies to survive infection.
The most important negative effect is the accidental creation of drug-resistant bacteria. These bacteria survive the initial doses of penicillin and reproduce, creating new bacteria with a higher chance of survival in the future. These resistant strains of bacteria are harder to kill, making the possibility of a drug-resistant pandemic more likely. The second most important negative effect is that penicillin kill all the bacteria they affect, both good and bad, and so they can cause damage to the body if the good bacteria are not replaced. This causes digestive problems, and new research suggests that good bacteria could be destroyed in the body forever from antibiotic treatment.