infectious

Something infectious is often about germs. It describes a disease that can spread from one person to another, like the flu or a cold. But it can also be used in a figurative way. Think of laughter. A fit of giggles can be infectious, meaning it quickly makes other people start laughing too.

The idea is simple: something infectious spreads easily. If enthusiasm is infectious, people seem to "catch" it from you. An upbeat mood, a big smile, or a catchy song can all feel infectious because they quickly affect the people around them.

The word "infectious" most often describes a disease that can spread easily from one person, animal, or thing to another. For example: "Measles is a highly infectious illness," meaning it passes easily between people. This is the main literal use of the word, especially in medical or health-related writing.

But "infectious" is also often used in an informal, figurative way. It can describe a feeling, mood, or behavior that spreads quickly among people. A laugh, a smile, excitement, energy, or enthusiasm can all be infectious. For example: "Her optimism was infectious; it lifted the spirits of everyone in the room."

The key idea is spreading. Something infectious passes from one person to another, either literally, like a virus, or figuratively, like joy moving through a group. In both cases, it suggests that the effect is quick and hard to resist.

You might hear people say a child's laughter is infectious, or that a performer's energy is infectious. In these cases, the word does not mean harmful; it simply means that the feeling spreads easily to others.

So, whether it refers to an illness or to a feeling that catches on, "infectious" describes something that spreads easily and affects other people quickly.