'TMZ Investigates: Obsessed & Dangerous: Hollywood’s Stalker Crisis' on YouTube

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 because she's scared one of them might break in and hurt her.

Sandra Bullock called 911 because of stalkers. Sarah Palin's eldest daughter, Bristol Palin, has been hounded by stalkers for years, and after scant help from the criminal justice system, she's taken matters into her own hands ... arming herself with guns.

Logan Paul was once in the crosshairs of a man who seemed obsessed with him -- and who even broke into his home -- and the guy ended up taking his own life.