Reikworm

Arguably worse than the Galloping Trots is the Reikworm. Any body of water can carry these organisms, and the different Imperial provinces have all kinds of bizarre methods that supposedly keep them at bay, including swirling water around the mouth six times before swallowing it in Ostermark, or never drinking untested water when Morrsleib is visible in Nordland. Whenever someone draws water from any unsecured source, there is a 5% chance that it carries the larvae of a Reikworm. Swimming in such water carries the same risk of contracting worms as drinking it. When exposed to contaminated water, Reikworm larvae will generally settle in the victim’s belly.

Description
When the tiny larvae of the Reikworm enters the body, it burrows deep into the guts of its host, where it feeds and grows. Soon, the host’s immune system is diminished, making him prone to catching other diseases. After three months, the worm will have grown to four feet in length and will be ready to head for the surface. The first visible sign of the parasite is a massive blister that forms on the surface of the skin. After a week, the blister bursts, causing an intense burning pain. Commonly, the afflicted plunge their hot wounds into cooling water for relief. If this happens, the mature Reikworm emerges and spits forth a new batch of young as a sickly yellowish fluid, which soon disperses in the water, ready to infect new hosts. After this, the worm then dies.

Source

 * Warhammer Fantasy RPG 2nd ED -- The WFRP Companion (pg. 28).