Disease was one of the few status effects in RuneScape. It was caused by being damaged by Zogres and Skogres in their dungeon near Jiggig, Diseased kebbits in Gu'Tanoth, 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), failing to cut a scrapey tree in Trouble Brewing, or by the disease handicap in the Dominion Tower.
The only skill that was not affected by disease is Constitution. All other skills could be reduced randomly by disease, taking away a number of skill points equal to the number on the disease splat. It only lowered one skill at a time and could lower the same skill multiple times in a row.
Wearing an inoculation brace provided protection against the effects of disease by preventing the reduction of skill levels, but it did not cure or immunise against disease. Alternatively, bathing in the thermal bath in Oo'glog cured and briefly immunised players against disease.
Skills would gradually recover on their own, but for an instantaneous recovery, potions from Herblore could have been used. Restore potions recover Combat skills except Prayer and Summoning, while Super restore recovers all other skills and a limited amount of Prayer and Summoning.
The effects of disease could have been countered with Relicym's balm. Drinking one or two doses would have reduced the effect, while three would have cured most disease altogether. Disease that was inflicted by scarab swarms and traps in the dungeon encountered in Dealing with Scabaras would have required four doses to be cured immediately. Instead of clicking the potion, a player could have clicked the now-yellow life points indicator to automatically drink their balm.
Bathing in the thermal bath in Oo'glog cured disease and temporarily immunised players against being infected.