K-pop has traditionally thrived on idol worship from the hundreds of thousands of young groupies who spend lavishly on concert passes, air tickets and merchandise.

It is these fans who propel performers from relative obscurity to almost instant super-stardom, the fans who will buy the products the stars are paid to endorse, the fans who will fill up venues with their ecstatic screams.

And K-pop stars know this — "fan meets" are held on a regular basis, managers keep in contact with fan clubs to make sure stars stay on the radar, and every concert or appearance is inevitably punctuated with "thank yous" and "I love yous" from the idols to their supporters.

"We are nothing without our fans," is probably the most common phrase heard from K-pop stars.

But what if some of these fans started sending letters written in menstrual blood to their boyband idols? Or began installing hidden cameras in their idols' homes and cars, stalking them so aggressively that car accidents resulted?

What if a fan decided she loved her idol group so much that she would "destroy" his rivals by doping another boyband's drink with poision?

All these horror stories are true and they are committed by a new breed of K-pop groupies gone wild — "Sasaeng", or "private" fans, whose quest to worship their idols have taken on horrifically disturbing proportions in the past year.

What started out as groups of teenaged girls banding together to support their K-pop idols has morphed into an ugly new phenomenon involving physical violence, blatant intrusions into personal privacy, and even sexual favours offered in exchange for information about the stars.

Who are they?

Sasaeng are usually female, starting as young as 13 to about 22, and they have made it their life's goal to make sure they are noticed by their idols, by hook or by crook.

Top K-wave stars targeted include male groups and stars like JYJ, TVXQ, B2ST, Jang Geun Suk, Big Bang, Super Junior, SHINee and SS501′s Kim Hyun Joong.

Korean managers speaking to Korean media have revealed that popular stars have between 500 to 1,000 sasaeng fans. On any given day, the stars have at least 100 full-time stalkers on their heels.

JYJ's three members have the most sasaeng stalking them (Getty Images) More

JYJ and TVXQ, in particular, have been the target of obsession. Here's a list of what they have had to go through in the past year:

-- TVXQ's phone lines are tapped and personal conversations recorded

-- Several sasaeng saved their menstrual blood and had it delivered to the JYJ members.

-- TVXQ's apartment was broken into and sasaeng attempted to kiss them while they were sleeping

-- JYJ's Yoochun had sasaeng fans who installed spy cameras in his parking lot and took pictures of him.

-- TVXQ's Yunho was poisoned by an anti-fan who gave him a drink with an adhesive mixed in. He had to have his stomach pumped.

Yoo Chun gets slapped by a sasaeng on his way out of a salon (Youtube screengrab) More

These examples are just a few of the ways sasaeng fans terrorise their K-wave idols.

These sasaengs' extreme behaviour extend to every aspect of their lives. They do not return home but instead usually sleep at PC Bang (Internet cafes) after a full day of stalking. Many skip school and some drop out completely.

Their lives become filled with every move their idols make. The sasaeng position themselves strategically around the homes or the management companies of their idols and communicate via Kakao-talk, the Korean version of Whatsapp, boasting about photos they are able to snap or reactions they are able to provoke from the stars.

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