The Rock Disease (岩のようになる病気, Iwa no Yō ni Naru Byōki, lit. "Disease That Turns One Rock-Like") is an affliction featured in the eighth part of the JoJo's Bizarre Adventure series, JoJolion. It is a hereditary illness passed down through the Higashikata family, and by extension the Joestar family. The bodies of those who suffer from this disease are known to begin hardening like rock, with symptoms beginning to appear at the age of 10 for males and in adulthood for females. As the disease progress, the victim loses more and more of their bodily functions until they eventually die. If the victim has a Stand, they may also find themselves unable to control its manifestation or abilities.
The firstborn of every generation in the Higashikata family suffers from this disease. Tsurugi mentions that a long time ago, the second son used to succeed the Higashikata or the family would adopt someone to serve as the next heir.
The earliest chronological example of the disease appears in the Shakedown Road arc during a flashback narrated by one of the inhabitants sitting at a bench. He recalls the rumors of Johnny Joestar traveling to America to recover the corpse of a saint in order to cure his wife of her affliction, using its ability to "remove" unwanted things and push them elsewhere. Although he succeeds, the sickness is instead pushed onto his young son, and so in a desperate attempt, he utilizes the Spin to transfer the disease to himself before being flattened by a stone under the effects of the newly developed Les Feuilles.
Later, Toyoko Higashikata discovers that the properties of the site of Johnny's death, which later hosts the Mutsu-kabe Shrine, allow the disease to be cured by having another member of the family pass the effects of the disease to themselves. The family adopts it as a tradition afterward.
It is Tsurugi Higashikata's fear for his inevitable development of this condition that motivates him to succumb to Yotsuyu Yagiyama's manipulation, as Yotsuyu demonstrates the ability to cure the rock disease with the use of Locacaca. However, this method remains imperfect due to the unpredictable and uncontrollable nature of the Locacaca's equivalent exchange, a problem not present in the altered Locacaca branch developed by the late Josefumi Kujo and Yoshikage Kira.
As Jobin Higashikata obtains and cultivates the new breed of Locacaca, Tsurugi's condition rapidly worsens as he experiences memory loss and his skin begins to melt. After Tsurugi reaches critical condition, Kaato Higashikata uses an injured Toru to perform an equivalent exchange between him and Tsurugi, using sap from the Locacaca branch to ensure the exchange benefits Tsurugi. Thus, Tsurugi is cured of the disease, which passes to a dying Toru. As Yasuho explains afterward, this marks the end of the disease as a whole.