My understanding is when referring to a type of thing in general we can either use the plural form or a singular with the definite article. For example:
The tomato is a fruit, not a vegetable.
Tomatoes are a fruit, not a vegetable.
But can an indefinite article also be used to make a generic reference? I saw this in Google's definition of "scabies":
A contagious, intensely itchy skin condition caused by a tiny, burrowing mite. Skin condition caused by a burrowing mite.
I found this definition kind of odd, because "mite" as a countable noun means an individual organism that belongs to the mite species. I thought the following would make more sense:
A contagious, intensely itchy skin condition caused by tiny, burrowing mites.
A contagious, intensely itchy skin condition caused by a kind of tiny, burrowing mite.
I am almost certain this question must have been asked, but I can't find any related questions. What are the applicable grammatical rules here? Also are there other examples?