I'd say hygiene is a construct. From that wiki article:

As mind-dependent objects, concepts that are typically viewed as constructs include the abstract objects designated by such symbols as 3 or 4, or words such as liberty or cold as they are seen as a result of induction or abstraction that can be later applied to observable objects or compared to other constructs.

With this in mind, hygiene itself cannot be seen directly, and thus abstract. We can see the effects of hygiene (such as a clean body, lack of body odor, or opposite of hygiene, such as athlete's foot or other diseases), but we cannot see hygiene itself.