Is there anyway to create categories of questions, like folders on the computer? For example: wholistic/natural products and food; physical activity and lifestyle; drugs for glaucoma; surgical/device interventions.
Yes, this feature is called "tags". When you create a post, you can add one or more tags to it. Any of the examples you gave could be tags. Tags should be short, limited to one topic and not include slashes. For example, "natural-products" or "healthy-food" or "lifestyle" could all be tags. (Please give some thought to any tags you create.)
Unlike categories or folders on your computer, where an item can usually only exist in a single category or folder, you can assign multiple tags to any post here. Tags have generally proven to be more useful than categories, and this approach has become something of a standard on websites.
With tags, anyone can find all the posts that were assigned to a tag by looking in the tags section -- see "Tags" in the top menu of the site (the top of any page).
For all questions about the features of this site, I created the "help-site" tag (and I used it on this post). Since the question was about categories, I also created the "help-categories" tag. For any new tags related to questions about using this site (or its features) I will make tags that start with "help-".
When you are on the main "tags" section of this site, you can use the search box to find tags you are interested in. For example, browse to the "tags" section and type "melatonin" into the search box. That will show the "melatonin" tag. If you then click that tag, you will see a list of all posts that were tagged "melatonin".
Please let us know if you have any questions or suggestions related to tags.