Rock Disease

"This curse on the Higashikata Family - your illness, I swear I'll cure it."

- Jobin Higashikata to his son Tsurugi,

The "Rock Disease" is a tentatively-named affliction that particular characters in JoJolion suffer from. It has been confirmed to be a hereditary illness passed down through the Higashikata family and by extension the Joestar family.

Description
The bodies of those who suffer from this disease are known to begin to harden like rock beginning at the age of 10. As the symptoms progress the victim loses more and more of their functions and memories until they eventually die.

The disease is a hereditary illness which always affects the firstborn of the Higashikata family. If the firstborn is a son, they will become symptomatic at age 10; daughters, however, have not had a specified age at which the disease starts to affect them. Rina Higashikata became symptomatic as adult, in her 20s.

Background
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 travelling 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 move 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.

Current story
It is mentioned that the firstborn of every generation in the Higashikata family suffers from this disease, but at some point the family has found a way to save the firstborn by having another member of the family utilize the equivalent exchange properties of the ground of their estate to take their place.

It is Tsurugi's fear for his inevitable development of this condition that motivates him to succumb to Yotsuyu's manipulation, exploiting the young boy into helping him set a trap for the protagonist. It is first hinted then and later expanded upon that the Locacaca fruit also seems to have the ability to 'cure' the Rock sickness, although its full effects have not been observed as the only known case was the dog Iwasuke.

Over the course of JoJolion, Tsurugi's condition rapidly worsens as he experiences memory loss and his skin seems to melt.