Stalkers have become a huge problem for celebrities ... and huge stars like Taylor Swift are scared of what some of these folks might do ... because sometimes things turn deadly. In "TMZ Investigates: Obsessed & Dangerous: Hollywood's Stalker Crisis," we explore the growing danger stalkers pose to celebs ... and we expose how laws and the police do little to protect stars from this sometimes deadly epidemic. Taylor's dealt with stalkers for years, and she keeps medical supplies in her home now