Learn English – ny difference between “unproductive” and “nonproductive”

While the terms are often used interchangeably with regard to illness, there are some differences.

Contract, in this context, means

[with object] catch or develop (a disease or infectious agent): three people contracted a killer virus

The term is regularly used with chronic diseases:

Women in their 30s and 40s are TWICE as likely to contract cancer than men - with breast cancer largely to blame

Daily Mail

Arthritis: What are the symptoms and who is likely to contract it?

compleatmother.com

Catch means

contract (an illness) through infection or contagion: he served in Macedonia, where he caught malaria

The principal difference is that catch suggests a transmittable infection, while contract can refer to a wider variety of diseases, including those that are not contagious.

Surely there should be more of a difference between the two words. If they mean the same thing surely there would be no need anymore for two words

There are a lot of near synonyms in English, so this does not follow at all. It's particularly so when you consider that act entered the English language in the late Middle English period, from French, while deed was in the language from the very beginnings of Old English. Many, but not all, seeming redundancies in English vocabulary have a similar origin story.

It is true that there are differences in nuances. The primary one would be that an act would generally be need to be relatively remarkable before we would use the term deed. One can do a particularly brave or noble or dastardly or evil deed, but one can't really do a lazy or everyday or common deed. There's nothing technically wrong with the latter, but it wouldn't be an idiomatic use.

There are similarly some differences in what verbs and prepositions are used with either. One doesn't do an act, one cannot be in the deed of anything.