This rating is due to the anime's content which includes:
* Graphic violence and gore: The anime features numerous scenes of violence, including zombies attacking and dismembering humans.
* Sexual content: There are scenes of nudity, implied sexual activity, and suggestive dialogue.
* Strong language: The anime contains strong language and profanity.
However, it's important to note that the rating can vary depending on the region. For example, in Japan, High School of the Dead is rated R15+, which means it is not suitable for viewers under 15 years old.
Ultimately, it is up to each individual to decide whether or not they find the content of High School of the Dead appropriate for themselves.