Junkie is a slang term for an addict. Its most common use is for drug addicts, but it has also been used to refer to enthusiasts of various topics, i.e. "TV junkie".
Drug junkies, those that rely on needles to administer the drug, have been "central to the spread of HIV/AIDS since the pandemic began more than 25 years ago." This is mainly due to the problem of sharing needles. They are also "major contributors to the U.S. crime problem".
The term originated from "junk," a slang term for heroin, and became popular in 1953 when William S. Burroughs released a book about drug addicts called Junkie.