What is he difference between a contagious and infectious?
Dictionaries do not make this clear, but perhaps it is because there is overlap in the meanings.
You’re right, the meanings overlap and the difference is not always clear. For every clear and unambiguous answer as to “the real difference”, there will be a response showing why it isn’t that simple.
“Contagious” generally means transmission is directly from one human to another human - no mosquito or other “middleman”.
One writer on Wikipedia suggests that “contagious” often turns out to mean that quarantine would be a reasonable and useful response.
In the medical context, “contagious” is a subset of “infectious”.
Contagious diseases are infectious by contact - i.e. you can be infected by the disease through contact with someone who is themselves infected. A disease which is spread through, e.g. contaminated water supply, or infected mosquitos or other insects or animals, would be infectious but not contagious.
Here’s a comparison of how the two words are used as attributive adjectives.
Adjectiving weirds language.
A contagious infection can be spread to directly to others. Like colds, flus, active tuberculosis.
But many infections are not contagious. Urinary tract infections, sinus infections, ear infections.