Penicillin is a medicine for infections caused by bacteria, the very small living things that can make you ill. It does not work on viruses, so it will not help a normal cold. Doctors use it for some throat, skin, or chest infections, but only when bacteria are the problem. It was the first famous medicine of this kind, and it changed the world: some illnesses that used to kill people could now be treated. In 1928 Alexander Fleming saw that a mold had got into a dish of bacteria, and the bacteria around the mold died. The medicine later came from that kind of mold. Though actually, turning that lucky dish into a safe hospital medicine took many other people and more than ten years. Some people's bodies react badly to it, so they cannot take it. Some bacteria have also learned not to die from it, so it does not fix every infection. You will hear the word from doctors and on medicine boxes. It is a normal, serious word, not slang.