How do you know if a part of your body is dead from infection?

I’ve personally felt invisible all my life, especially when a friend turned out to be fake and didn’t seem to take any type of interest in what i said or any type of input i said. being invisible is a terrifying feeling to experience due to the fact you feel like no one sees you and just sees you (sorry if that doesn’t make sense). it’s like a deep emptiness that no one can fill, just you; you have to fix that emptiness by yourself and you don’t really want , or in some cases want to be invisible. i can see many invisible people when they intentionally stay away from people that can potentially hurt them in any way. also, when a person intentionally breaks off a connection so they aren’t seen by people.hope it