The Difference between 'Contagious' and 'Infectious'

The difference between contagious and infectious can be difficult to figure out because there is some overlap in meaning.

An infectious agent is something that gets you sick. An infectious illness is one that spreads by an infectious agent entering the body. A contagious illness is one that is spread by coming into contact with someone who is sick (who is infected with an infectious agent).

Something contagious (such as a virus or bacteria) is spread from one person or animal to another by touching or by coming into contact with another person or animal's germs.

All contagious diseases are also infectious because if you can catch it from someone then their germs (viruses or bacteria) are being passed to you. But not all infectious diseases are also contagious. Infectious illnesses are spread by an infectious (=infection causing) agent (such as a virus or bacteria) entering the body, but some infectious illnesses, such as food poisoning, cannot be spread to another person.

For example, a cold is contagious and infectious: an infectious agent (e.g. a cold virus) enters the body and is passed to other people by contact (e.g. shaking hands or kissing). Food poisoning is only infectious: an infectious agent (e.g. bacteria) enters the body but you cannot spread food poisoning to other people.

There is another common use of these words. Contagious and infectious are often used figuratively. Both words are used to say that something (not a virus or bacteria) can be spread to other people: A laugh or a smile can be contagious/infectious. For example, you can say "She had an infectious laugh" (=her laugh made other people around her laugh, too).

I hope this helps.