Five Tips for Creating a Fictional Disease

A beautiful tree in Davis, CA. Many people study disease at UCD. Photo by Kate Ota 2015​

Books and movies love disease. Whether it’s the crux of the story, like a zombie-fying outbreak, or it’s background to other elements, like in Marie Lu’s Legend. Sometimes we need to include a real disease in a historical novel, or a fake one in fantasy or sci-fi. For a real disease the method of how to write it is clear—do the research and maybe read some other books (novel or non-fiction) that include it. But to make one up is a whole other ball game. I took many courses in undergrad and grad school that discussed disease from all angles. No need for you to take an epidemiology course though, I’ve gathered my knowledge and have the top five tips for how to make up a disease right here!
 
Determine a Method of Transmission
For most zombie stories, the method is a bite. But there are many ways to catch a disease. Here are the main methods:

Person-to-person is when body fluids (wink wink) are exchanged to transmit fluid. Transmits slowly through a population, but tends to be pretty infectious. Examples: HIV, Hepatitis B, syphilis, and other STDs.

Droplet transmission is exactly what you think it is. Jessica has a cold, sneezes in your face, you breathe in her mucus droplets, and now you have a cold. These tend to transmit quickly through a population, and can range from mild to deadly. Examples: common cold, flu, tuberculosis.

Airborne or contaminated object transmission is similar to droplet transmission, except you don’t need to be close. These diseases can linger in the air or on a surface long after someone has left the room. These are also often quite contagious. Example: measles.

Food/water borne diseases are exactly what they sound like. Often, they cause digestive symptoms and place like the FDA monitor for them. These are often killed in proper cooking or boiling processes, so they tend to have a class divide. Examples: E. coli, cholera, botulism

Animal-to-person contact and zoonotic diseases tend to be rarer now that most people only interact with household pets instead of farm animals or game. Many diseases stay in animal populations and rarely make the jump to humans, like rabies. A more common disease to crossover is Toxoplasma gondii, which is transmitted by cat poop. Humans tend to be very bad at fighting these diseases because the germ evolved with a different host. Therefore, they can be very deadly to humans while natural host animals survive better. Another example: black plague (rats/prairie dogs).  Some diseases begin as animal-to-person, then can be transmitted person-to-person, like Ebola.

Insect bites (aka vector-borne diseases) transmit many problematic diseases. Mosquitoes, fleas, and ticks are the main offenders to humans. Often, the insects pick up the disease from another animal host. Ticks get Lyme disease from infected deer and mice, then introduce the Lyme to humans when they bite us later. Insects like warmer climates, so vector-borne diseases are more of a health hazard in the tropics than in arctic zones. Other examples: malaria, West Nile virus, Zika
The environment also hosts many disease-causing agents. Parasites like ringworm, hookworm, Legionaries, flesh eating bacteria, the list goes on and on. Humans catch this by being exposed to dirty water, soil, etc. especially with open wounds. As you may have guessed, rates of these diseases also tend to have a class divide.
 
 
Morbidity and Mortality Rates
Once you have your method of transmission, you should determine the morbidity and mortality rates. Morbidity is the state of being sick. The morbidity rate is how often people are catching it, you can think of this as representing how contagious it is. Mortality is the state of being dead, so the mortality rate is how often it kills the person it infects. You can think of this as representing how deadly this is. Plague-level events have both high morbidity and high mortality. Something like rabies has low morbidity, because it rarely happens in humans, but once contacted it has high mortality. The common cold has high morbidity in the winter, but very low mortality, as few people will die from it.
In your writing, you should determine how present you want this disease to be. If your fictional disease has high morbidity and mortality rates, it better be the main plot focus. Otherwise, it may be too distracting for your reader. Consider how it is transmitted in determining your rates, see the section above.
 
Symptoms
Symptoms of a disease are your body trying to fight it off. Fevers are from your body trying to boil that disease to death. Snot is from trying to get that disease out of your nose. Coughing is to get rid of the mucus that trickled into your lungs. On and on. You should think about what symptoms your fictional disease has and why. Tie the symptoms together, like high blood pressure causing headaches, a rash leading to sores, runny nose leading to sore throat. And have the symptoms escalate through the course of the disease. This is not only realistic, it adds drama to the story, and helps your characters keep track of how close a comrade may be to death. (We only have until she develops a fever to find a cure! After that, she’s a goner!)  
 
Scars
Not all diseases leave no trace. Smallpox left people with large scars on their skin. Ebola can change a survivor’s eye color (and continues to live in the eye!) Scarlet fever can weaken the heart. Chicken pox can come back as shingles. Malaria never really leaves you. If anyone is able to survive your fictional disease, consider the after-effects that may remain. It may be a somber reminder of what they endured, but it could also have social consequences, like facial smallpox scars.
 
Cure
If the plot or a subplot revolves around developing a cure, then you’ll need to figure out a cure. Even if you have a scientist pop up and do a little hand waving, you’ll want to know more than just, “you need this injection! Ta-da!” There are several places in the cycle of disease transmission in which you can stop the disease. Let’s discuss those:

The agent: Identify the disease agent; a virus, bacteria, parasite, fungus, and discover what will kill it outside the body. Bleach? A bacteriophage? Gotta identify the problem before any other solutions are found. Identifying the agent is usually early in the story. The exception is a fantasy with a historical world, pre-Victorian era or earlier, in which germ theory was not developed yet. In that case, this step and many others, may not apply.

The reservoir: This ties back to how it’s transmitted. Once that’s known, the reservoir can be handled. If it’s on dirty surfaces or in the water supply, those can be cleaned. Animals can be treated, insects can be sprayed for. If it’s person-to-person only, we’ll get back to that. Take note that hospitals will often adhere to the assumption that everything is contaminated and can transmit the disease and will be cleaning through the process of treating patients. But if you’re writing a fantasy with a historical flavor to it, maybe hospitals don’t apply to you.

Portals of exit and entry: How does the disease get out of one person (portal of exit) and into another (portal of entry?)  Once that is found, your characters will know if things like face masks, gloves, hand washing, and different levels of personal protective equipment are helpful. For anything particularly infectious and deadly, hazmat suits might be used before this information is available and once this is known, the medical teams will step down their equipment.

Susceptible hosts: The disease can only keep going as long as someone is susceptible to it. Part of disease treatment is prevention, often an immunization is developed to protect those who haven’t been infected. But it could also be as easy as hand washing and basic hygiene, like in common cold prevention.

A cure: Maybe you want a real cure that can help those who already have the disease. This will be dependent on all the other factors you have developed. Maybe you’ll choose a vaccine for your virus, an antibiotic for your bacteria, a substance toxic to the parasite, or maybe your characters will treat the symptoms until it passes. It all depends on the rest of the world building you’ve done to this point. If the disease’s mortality rate is high, the cure will be preventing it—sorry to anyone who caught it. If the mortality rate is especially low, there may be no pressing need for a cure. Just remember that the cure needs to kill the disease agent without killing the host.
 
Need more inspiration? Choose a morbidity rate and mortality rate to mimic and search for similar diseases. See how different organizations handled it, from Venice inventing the quarantine for ships with possible plague to how the WHO contains Ebola outbreaks.
 
Found any of these tips helpful? Have any fresh ideas to add? Let’s discuss in the comments!