In the introduction to Insults & Injuries, there's a few words about "what this book is not." Among other things, we tell the reader that I&I does not contain rules for creating new diseases with balanced DCs, and the reason for this is given: in real life, disease isn't game-balanced. In a usual game, you would expect the DCs of a disease to vary with how dangerous it is in one way or another. For example, you might expect that a disease with few or no severe impact could have either a very high DCs, because the consequences of failure are small, or a very low DC, in correlation with its deadliness. The major determining factor there would probably be whether a GM intended to inflict said disease on low level characters, who are oh so amusingly breakable, or high level high level characters for whom twenty points of Constitution damage might not be such a big deal. Unfortunately, that isn't how real life works.

Here are some examples. One of the services where I currently work is a psychiatric consultation team covering a geriatric ward. Patients with medical problems, dementia, and general poor health get admitted under the geriatrics team -- a family doctor or an internist -- and when those patients have psychiatric disorders or their mental health is thought to be a factor in their illness, my team gets consulted to give an opinion and offer concurrent care. As I write this, that ward is currently under partial quarantine due to an influenza outbreak; workers and visitors can still come and go on the ward as needed, but everybody has to wear a gown, gloves, and face mask at all times and every patient on the ward is receiving preventative antiviral medication. Why did this flu outbreak happen? There are a few different ways to look at this. One factor is that a geriatric ward is unsurprisingly full of elderly patients who aren't in the best of physical shape; it's reasonable to assume that their Constitution scores have gone down with age, and their Fortitude saves with them, which might make them more vulnerable to the flu. Sadly, we can't just peek at my patients' character sheets and see what their actual Constitution scores are. Another factor is that this could be a particularly virulent strain of the flu; it so happens that the whole city is experiencing a wave of influenza right now and my ward is not the only one in the hospital that's currently affected. So, either we have a population with low Fortitude saves, or we have a disease with a high save DC, or both. Fortunately, it's not a particularly dangerous flu; some of the people on the ward are very sick, but to my knowledge none have died of it (which could say something about the disease or something about the Heal checks made by the doctors and nurses). In any event, there's a host of other factors at play, but that's keeping it fairly simple. One way or another, the game balance of the disease seems questionable.

Here's another example: HIV. Say that a character gets exposed to HIV. First off, there's approximately a one in ten chance that the exposed creature is simply immune to the virus, which is perfectly reasonable in game terms. Next, there's the percent chance of infection per exposure, which we can think of as being analogous to the Fortitude DC. Let's ignore blood transfusions and intravenous drug use, which operate by their own special "rules." The risk of "catching" HIV from any sort of sex act varies tremendously with what the particular act is, but it's estimated to never be higher than 0.1%, meaning that statistically, fewer than one in a thousand people engaging in an at-risk behaviour one time will be infected. As I write this, I realise I actually have no idea if this statistic includes the probability of someone being genetically immune, but that's not important... the point is, the risk from a single exposure is low. A low chance of infection would presumably mean a very low save DC; a level one character making two Fortitude saves to avoid infection should fail them both less than one time in a thousand. If someone does become infected, however, the save DCs skyrocket, to the point that before effective drug therapies were found, HIV was almost universally lethal within a very short time, and it's worth remembering that in the 1980's when the disease first emerged, it was young, healthy people (of what might be considered "adventuring age") who were dying the fastest. So, put it all together, and you have a disease with a low initial save DC, a high later save DC, and rapid, serious consequences of infection. That actually sounds fairly well-balanced doesn't it? It would be, except in real life, your Fortitude save doesn't scale up the way it does in most games. In a game, by virtue of its later high DCs and its lethality, a virus like HIV would probably be one that gets used against higher-level characters, and in reality, it hit people of all sorts of experience levels equally. In fact since the highest-risk population is young, impulsive people who don't always remember to protect themselves (sexually or during drug use), the virus arguably targets low-level characters preferentially.

Other examples? Meet a child with cancer and tell me that seems game-balanced to you.

And that's the crux of it. In games, we get used to the idea that things should be fair. We get frustrated with GMs who throw unbalanced combat at us or don't give treasure appropriate to our level. Disease, more or less by definition, isn't balanced or fair. That's why it's scary.