Antibiotics

Antibiotics are chemicals released by one microorganism that, in small amounts, kills or inhibits another bacterial growth. For example, the antibiotic penicillin (derived from the Penicillium mold) was initially used to treat staphylococcus, but is now used to treat gram positive infections such as streptococci bacteria.
Antibiotics are chemotherapeutic agents, which are drugs intended to eliminate a disease organism. Ideally, the chemotherapeutic agents are also magic bullets, which means that it damages the pathogen but no the host. For instance, penicillin affects the murein in the cell wall of a bacteria, which won't harm humans because they don’t have murein or even cell walls. This makes it a magic bullet. On the other hand, cyprofloxacin works on DNA synthesis, which inhibits bacterial growth. This also inhibits the DNA synthesis in humans, which can cause some side effects, especially in kids. This means that cyprofloxacin a chemotherapeutic agent but not a magic bullet.