There are a number of stories that have infectious diseases play a major role in the plot. It’s a great plot device, if used well—it forces isolation, people to work together, fear, panic, and death. The problem comes when authors don’t understand how diseases work. It is true that fantasy author do have more leeway when it comes to what they can do with their diseases, but there are still some basic points that should be kept in mind.
Infectious diseases come in the form of either viruses or bacteria. In fantasy, magic is a third choice, which will be covered later. In terms of general writing, unless you’re writing a medical thriller or something of the sort, you don’t need to know a whole lot about the differences between the two. There are some differences that are important to note, especially if you are writing about a time since the invention of vaccines and/or antibiotics. Vaccines exist for both viral and bacterial pathogens. That being said, some of the early vaccines were for viruses; the first vaccine was for smallpox. They are generally used as preventative measures to keep people from getting certain diseases, and some may require boosters to stay effective. Antibiotics (or antibacterials), on the other hand, only work for bacterial diseases, and they either kill the bacteria or inhibit its growth. This means that they will not work on things like the flu.
If your invented disease is magic, clearly these rules don’t apply. What that means, however, is that you shouldn’t have your doctors using antibiotics to fight the magical disease. They can attempt it if they don’t know that it’s magic, but as soon as they figure out that it’s not bacterial, they should not try to use it. Similarly, it would probably be incredibly difficult if not impossible to vaccinate against something caused by magic.
Infectious diseases spread in certain ways. This doesn’t mean that all diseases spread the same way. A few possibilities are as follows: though physical contact, though ingestion of infected water or food, through sharing of fluids, or through the air. Many of these are obvious. For sharing of fluids, this can include blood, seminal or vaginal fluid, or saliva.
Magical diseases are, of course, different. For one thing, as opposed to regular diseases, magical diseases can be set to specifically target or spare people, depending on the magic system of your world.
Drug-resistant bacteria are a real threat. To spare you the details, the more an antibiotic is used, the more likely it is that large sets of the bacteria will become resistant to it. This can appear in a number of different ways, with bacteria being resistant to varying numbers of drugs. Some are only resistant to one can be treated by simply switching to another drug. The bigger problem comes with things like XDR-TB (extensively drug-resistant tuberculosis), where the strain of TB is resistant to not only the first line drugs but also the second-line drugs.
Vaccines aren’t 100% effective. Boosters (getting a second or third shot of the same vaccine) help with this by boosting the immune reaction to the virus or bacteria, but even with this, there is never a 100% guarantee that someone who received a vaccine cannot catch the disease. Along with that, not everybody can receive vaccines for various reasons. These reasons can include weakened immune systems, such as those of people with TB or HIV/AIDS, as well as people with egg allergies. The second reason is because many vaccines are harvested in chicken eggs. Herd immunity helps with this problem. The basic idea behind herd immunity is that, if enough people have immunity, the disease won’t spread to those who don’t have immunity.
There are many controversies around vaccines and around disease prevention or treatment in general. In some religions, there is a feeling that vaccines and the like are circumventing God’s work and that, if God wanted someone to live or die, it should happen without other people getting involved. In some Muslim countries or communities now, there is a feeling that American doctors who are providing vaccines are instead trying to sterilize them.
Not all cultures are or always have been as knowledgeable about diseases as first-world countries are now. This may seem obvious, but it is important to remember for writing about any time or place with different levels of scientific knowledge than ours. Words like “bacteria” and “virus” might not be used. Vaccines may or may not exist, depending on when and where you’re writing about, as they were first invented (or at least finalized, as I’m using the introduction of the smallpox vaccine as the start date) in 1798. There have throughout the years and cultures been thoughts that bad smells, an unbalance of the humors, the theft of the soul by an evil spirit, or numerous other causes to have been the cause of disease.
If you are thinking of implementing a specific disease into your story, you should obviously research that disease and how it works at more depth. The important thing to remember is that you should attempt as much as you can to get this correct. Magic is an okay excuse if you have magic as the reason for your disease, but you should still know about how diseases work before writing about them.