Diarrhoea is a tier 3 symptom, unlocked after Vomiting or Pulmonary Oedema. The pathogen becomes more active in the host's digestive tract, allowing transmission through feces. The loss of water can cause fatal dehydration. Poor countries become 0.05% more vulnerable to infection.
Along with Fever, Inflammation, and Tumours, Diarrhoea has a chance of becoming better understood by scientists in a pop-up message. Evolving Diarrhoea after this will allow scientists to exploit it to develop a cure faster.
When combined with the Sneezing symptom, the Oops symptom combo is formed, which slows cure research and draws more attention to the disease. If combined with the Insanity symptom, the Public Defecation symptom combo is formed, which increases infectivity but draws more attention to the disease.
Note: In real life, diarrhoea is not usually fatal, except in extreme cases where the victim is not drinking enough fluids. Most people do survive diarrhoea in real life.