A biohazard is something living, or from a living thing, that can hurt people’s health — a dangerous virus, a kind of bacteria, infected blood, or waste from a hospital lab. You will know the idea from the orange warning sign with three curved claws; that sign means “this could make you ill if it gets into you.” Labs, hospitals, and waste trucks use the word. It is serious and official, not a joke word in careful speech, though people do stretch it in films and games for anything “gross and dangerous.” Though actually, a biohazard is not the same as a poison bottle or a radiation sign: the danger is biological, not a chemical burn and not radioactivity. Bio- here means “from life,” and hazard means “danger.” Something can be a biohazard even if you cannot see it.