Is That Cold 'Contagious' or 'Infectious'?
What to Know
Contagious diseases are spread by contact, while infectious diseases are spread by infectious agents. Something "contagious" is by default "infectious" because contact exposed you to the infectious agent, but something infectious isn't always contagious. You can be infected with food poisoning, which itself is not contagious.
Flu season descends annually, and we wonder whether our coworkers who come down with it are going to pass it along to us. If they do, is it because they're contagious? Or because they're infectious? What exactly is the difference?
Contagious vs Infectious
The crux of the distinction is this: contagious diseases are spread by contact, while infectious diseases are spread by infectious agents.
So, if contact with your sniffling coworker results in your developing the sniffles, your coworker's sniffles are contagious. (If your coworker's sniffles are caused by allergies, you can't catch them because allergy symptoms aren't contagious.)
But here's the thing: your coworker's contagious sniffles are also infectious.
Huh? Yup. Infectious sniffles are sniffles that are spread by infectious agents; an infectious agent is the thing that gets you sick, usually a virus or a bacteria.
Anything contagious is always automatically infectious: if you can catch it from someone, it's being passed to you via an infectious agent.
But the reverse isn't true. Just because something is infectious does not mean it's contagious. Food poisoning is a good example of something infectious but not contagious: food can be contaminated with a bacteria that makes you sick, but you can't give your food poisoning to someone else just by shaking their hand or even giving them a kiss.
Figurative Use of Contagious and Infectious
Both these words are also used figuratively, often in much happier contexts. Your sniffling coworker might also have a contagious laugh or an infectious enthusiasm. While both words are used of both pleasant and unpleasant things, contagious is more often chosen for the unpleasant, as in "grumpiness that's contagious."