Infectious describes something that spreads easily from one person or thing to another. When a disease is infectious, it can pass from a sick person to a healthy one through contact, breathing the same air, or touching contaminated surfaces. The common cold, chickenpox, and flu are all infectious diseases.

But the word goes beyond illness. When something is infectious in a positive way, it means it spreads naturally because it's so appealing or energizing. Infectious laughter makes everyone around start giggling too. An infectious smile brightens the mood of a whole room. When your friend's enthusiasm for a new hobby is infectious, you find yourself getting excited about it too, even if you weren't interested before.

The word captures this idea of contagion: something moves from person to person almost automatically, like a spark jumping between dry leaves. A yawn can be infectious. So can fear, joy, or confidence. Teachers hope their love of reading will prove infectious in their students. Athletes on great teams often talk about how one player's energy becomes infectious, lifting everyone's performance.

Whether describing germs or emotions, infectious suggests a natural, powerful ability to spread and multiply through a group.