Fictional diseases can be a great way to add depth, complexity, or conflict to a fictional world. In fantasy worlds one may find lycanthropy or vampirism, while in apocalyptic fiction perhaps a zombie virus runs rampant. In game-oriented worlds diseases may act as a status-altering game mechanic, while in narrative worlds sudden illnesses can be used to drive a plot or provide a crisis that must be solved. Even as a minor element (a magical cold causing spell-casting sneezes), the addition of a fictional disease is one that should be considered by all worldbuilders.
Designing Fictional Diseases
Most worldbuilders are concerned first and foremost with the symptoms of any fictional disease. While most real diseases commonly cause symptoms that are inconvenient but generally not deadly (fever, fatigue, coughing, congestion, and/or rash), fictional diseases typically focus on extreme symptoms. Disease-causing transformations (such as those that occur in lycanthropy, vampirism, and zombie viruses) may exist, and diseases are often lethal if left untreated (as this can be used in narratives to create tension). With magical worlds, symptoms can involve magical elements and diseases may cause levitation, invisibility, memory loss, loss of magical ability, or other fantastic effects. Boils, open sores, rabid behavior, delirium, and other symptoms may also be common.
In fictional worlds, disease symptoms may only manifest themselves at certain times, such as during the night or during a full moon.
The question of who in a group is infected and who is not may be an important one.
Mode of Transmission
The mode of transmission of a fictional disease is an important part of design. Many fictional diseases are spread by saliva or bite. Real world disease can be caused by direct contact (person-to-person or animal-to-person), indirect contact (contact with a contaminated item or surface), insect bite, or food/water contamination.
Real disease is caused by viral, bacterial, fungal, or parasite infection. In fictional worlds there may be other sources causing disease. These fictional causes may be magical or technological in nature.
Cures & Treatments
There may be no cure for your fictional disease. The discovery or development of a cure may drive your narrative. There may be folk medicine or potions that may or may not work. There may only be treatments that keep the worst of the symptoms at bay but do not entirely irradicate the disease. If there is a cure, it may need rare or expensive ingredients (and may lead to missing ingredient quests in game-oriented or narrative worlds). When designing a fictional disease it is equally important to consider the cure.
Impact on Society
The way that people respond to a disease can be heavily influenced by their culture and beliefs. You should consider the cultural impact of your disease, and how it is viewed by different groups in your world. For example, some cultures may view the disease as a punishment from the gods, while others may see it as a test of faith. The infected may be treated with care at hospitals, or may be sent to disease colonies or quarantine zones.
Diseases can be used to explore themes of corruption within our broader society, or examine how we treat our most vulnerable.