system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is a nightshade? Why do we consider them different to other fruit/vegetables?

I often hear about people avoiding nightshades in various diets, but I'm unsure why. I also can't seem to piece together what they have in common, or why that might be important.
gpt
"Nightshade" is a huge family of plants that covers everything from tomatoes to potatoes. And belladonna too, commonly called "deadly nightshade". 

The family as a whole is not considered to be different than other fruits or vegetable. 

The general reasons why anyone would try to avoid anything are "medically diagnosed allergy", "self diagnosed allergy", or "fad diet that they got suckered in to".