Disease is one of the few status effects in OldSchool RuneScape. It is caused by being damaged by Zogres in their dungeon near Jiggig, Skogres and Fever spiders, from the scarabs during Dealing with Scabaras in their dungeon in Sophanem, by failing to pick the lock on a zogre coffin (again, in dungeon near Jiggig) or failing to cut a scrapey tree in Trouble Brewing.
All skills including Hitpoints may be reduced randomly by disease, taking away a number of skill points equal to the number on the disease splat. It only lowers one skill at a time and can lower the same skill multiple times in a row.
Wearing an inoculation brace provides protection against the effects of disease by preventing the reduction of skill levels, but it does not cure or immunise against disease.
Disease will go away on its own after several minutes. Skills gradually recover on their own, but for an instantaneous recovery, potions from Herblore can be used. Restore potions recover Combat skills except Prayer, while Super restore recovers all other skills and a limited amount of Prayer.
The effects of disease can be countered with Relicym's balm. Drinking one or two doses reduces the effect, while four cures most disease altogether. Instead of clicking the potion, a player may click the now-yellow hitpoints indicator to automatically drink their balm.
Another method is teleporting to Clan Wars, using a Ring of Dueling. Go in and out of the free for all portal.