Fifty years ago, people didn't know to avoid bad cholesterol. To avoid is the infinitive

Question

is it being used as an adverb, adjective, or a noun?

Answer 1

Noun; it is an act of doing something, which is avoiding in this case.

Answer 2

It's a noun, used as the direct object of "did know."

Answer 3

Thank you very much

Answer 4

You're welcome.