Highschool of the Dead premiered nearly 12 years ago in Japan. The anime, which follows the journey of a group of teenagers attempting to survive a zombie apocalypse, was generally well-received by critics, and has now become infamous for its graphic violence, strange sense of humor and blatant sexualization of its characters.

For those who weren't around at the time, or haven't had the chance to research the series, it can all seem rather confusing. However, after only a single season (which ends on a cliffhanger), many fans have wondered what on earth happened to prevent this NSFW series from being renewed.

RELATED: Bocchi the Rock! vs. Zombie Land Saga: Which Is the Better Starving Artist Anime?

Highschool of the Dead's Early Volumes Rivaled Naruto for Popularity

Written by Daisuke Satō and illustrated by Shōji Satō, Highschool of the Dead debuted in the September 2006 issue of Fujimi Shobo’s Monthly Dragon Age magazine. By 2007, Shobo and Shoten began to produce tankobon volumes of the released chapters, and continued to do so until 2011. By the time the seventh and final tankobon was released, the manga had more than three million copies in circulation in Japan. The series became so popular online through the use of scanlations that its authors included an English message within the magazine’s printing of the fifth chapter, requesting that readers buy the manga when it's available.

When Highschool of the Dead was released in English, its second volume climbed to the fourth spot on The New York Times’ Best-Selling Manga List, beating out such classics as Naruto and Blue Exorcist. It continued to be well-received in Japan, and repeatedly appeared high on the Oricon sales charts. To put it simply, a lack of sales or interest was not the reason for Highschool of the Dead’s untimely end. Both the manga and the anime were huge successes, and fans were looking forward to what would come next.

Unfortunately, the manga's writer, Daisuke Sato, became ill in 2008, making the production of the series increasingly difficult to manage. That led the manga to be put on hiatus until 2010 -- the same year Highschool of the Dead's anime adaptation aired in Japan -- although even upon the writer's return, little else was created. In April 2013, Highschool of the Dead received a brief comeback through the release of a single chapter; however, none of the main characters was present. Instead, the chapter showed police clearing the school and a group of other survivors contained within it.

RELATED: One Piece, Bleach or Naruto: Which Big Three Series Has the Best Shonen Girl?

The Passing of Daisuke Sato Marked the End of the Show

Saya Takagi pointing in High School of the Dead.

Sadly, in 2011, Sato was diagnosed with ischaemic heart disease, and his condition grew progressively worse over the next several years. Shortly after the release of Highschool of the Dead’s now final chapter the manga was placed on hiatus once again. Sato passed away from in 2017 at age 52.

In 2019, Highschool of the Dead co-creator Shouji Sato was interviewed by Comic Natalie, where he addressed the death of his friend, and why the series was not going to continue. “Overseas fans who don't know the details of the situation say, ‘draw a continuation right away,’ but it's not that easy," he said. "We are saddened that the work has stopped halfway through, but just thinking about Daisuke Sato makes me think that I cannot carelessly get involved.”

While fans are undoubtedly saddened that Highschool of the Dead won't be revived any time soon, it's honorable that Shouji Sato views Daisuke as being an integral part of the manga's creation; it's a respectful decision to keep the series as part of his legacy. Although the story of what happened to this series is unpleasant, fans can still go back and read the manga, or watch the 2010 anime. It remains just as bizarre and controversial as it did back in 2010 and is worth a watch for anyone who hasn’t already experienced it.