Contagious strictly means spread through contact, while infectious means spread by germs more broadly, including through the air or water, not only by touching. In everyday use, though, both words also describe a mood, laugh, or enthusiasm that spreads quickly, and there they are close to interchangeable.
The core difference
Contagious implies spread by contact; infectious implies spread by germs generally, contact or otherwise.
- contagious — capable of passing from one person to another through contact; also, quickly spreading to others: the flu is highly contagious during the first few days of symptoms.
- infectious — able to be passed from one person or animal to another, usually through germs; also, quickly spreading to affect mood: chickenpox is highly infectious among young children.
How to tell them apart
In medical usage, contagious points to diseases that spread through direct or close contact, while infectious is the wider term, covering anything caused by an infecting organism, whether it travels by touch, air, water, or another route. That is why a doctor might call a disease infectious without calling it contagious, but rarely the other way round.
Outside medicine, both words happily describe something that spreads through a group without any germs involved: a contagious laugh or an infectious laugh both mean the same thing, and either is a fine, natural way to describe enthusiasm or a mood catching on.
