I think maybe it's worth commenting here that we are very good in the gaming community at imagining how physical harm might be done with spell or sword, but pay much less attention to the other minutia of daily existence for human (and demi-human) beings. How often do your PCs have to deal with things like allergies, crummies in the tummy, a bad head cold, diseases acute and chronic, and that sort of thing.
While I recognize that many systems account for these things, as well as similar effects engineered with poisons, it has not been my experience that these things really make it into gameplay, unless of course it's as an adventure hook of some kind (e.g., Plague Rats of Nurgle, or some such thing). I think there are some good reasons not to include them. They're mundane, for the most part, and kind of depressing. They kill the whole heroic vibe. Conan with explosive diarrhea is just not as awesome, somehow. Oh, good. Now I'm going to have to live with that image in my head. You will too. You are welcome.
In some video games (Skyrim comes to mind), you can get diseases, of course. Then you simply go to Ye Olde Potion Shoppe and buy a potion and *poof* you're cured. That's not really a very difficult effect to overcome, except possibly at very low level, when you're pretty much broke most of the time. Once you have a bit of gold, it really isn't much of a big deal at all. That seems sort of a wasted effort, to me at least. If diseases don't actually do much harm, and if they're so easy to cure (and instantaneously!), then what really is the point of have them as elements of gameplay?
On the other hand, what if you have diseases that are dangerous, contagious, and either difficult or impossible to cure without specialized help (like a cleric of some god of healing/purity, etc.). That is, what if not every healer is able to heal them? What if there is an object or substance that is required to cure them? Then things are more complicated. That creates a quest. That could result in some neat gameplay.
However, that doesn't really get at the more mundane forms of disease, like the common cold, not to mention such diseases as are common, uncomfortable, but generally not deadly (e.g., chicken pox). As anyone probably knows, a bad cold can lay a person out for days at a time. Some forms of the flu are very, very dangerous. If an adult catches some childhood diseases, it can be deadly or can lead to side effects like sterility. So, these sorts of things can have short-term effects on the person's ability to do routine things. They might affect the person's overall state of health, and thus their ability to deal with new problems that affect one's health.
When you GM a game, do you ever use diseases in these more mundane ways? Do your PCs ever get a bad cold? Do they get STDs if they go out whoring? Do they suffer from hangovers from overindulgence in alcohol (or the Purple Meat, for that matter)?
For myself, I'd have to say I'm on the fence about these things. Yes, they add more "grit" and "grim-dark," and more "realism" to the game. Fine. But I also worry that they're just another book-keeping nightmare for the GM, or that players might just see such things as the GM being a dick (and just to be a dick). So, I tend not to use diseases and other disease-like physical effects. Perhaps, though, I've been wrong to do this.
Diseases are often terrifying. They can be deadly. They can become the center of one's universe until (and unless) one recovers. Even when that happens, there can be long-term effects. What if, for example, polio was a thing in your game world? That could create all kinds of problems for PCs. What if epidemics of influenza, cholera, smallpox, The Plague, etc., were common, or at least real threats to the PCs and their efforts in the game world? There's another potentially interesting way to create an interesting in-world effect.
Further, what if the PCs themselves become vectors of the disease in question, either because they are affected or because they are carriers of it (Thyphoid Mardok the Barbarian)? More potential in-game complications would be possible in such a situation.
So, to sum up, I think I'd like to use more diseases in my games, but not as short-term, forgettable, or otherwise inconsequential factors, but as part of the game world itself. This would include in-game effects on the PCs of course (e.g., stat drain, penalties to actions and/or saves, long-term effects on the PCs' physiques, etc.). It would also include things like determining if, how, and for how long outbreaks of particular diseases would occur. It could also, and I think this is the really interesting part to me, make consequential the choice of where one eats, drinks, and sleeps. Why pay for an inn that is of higher quality? Because eating bad food with filthy peasants can make you sick or dead. That's why the extra coin is worth it. When the choice of an inn is (potentially) a life and death choice, then it makes even mundane in-game decisions consequential.
If you include other things that could affect characters, like lack of sleep, saddle sores from long rides, the consequences of marching long distances in worn-out boots, failure to launder one's clothing resulting in infestations of nits and fleas, getting the drizzling shits from bad water or food, then all of a sudden you've made the world a deadlier, dirtier, and less pleasant place. You cleric becomes more than a healer with a club. He or she must also consider healing and health in a more holistic fashion (avoiding disease is a good way to avoid having to heal it). Players then become a whole lot more particular about things that normally get hand-waved in many games.
I suppose that getting things to that level of detail could slow down play or make the world to much like the real world for comfortable assumptions to rule the day. Hell, I may not even do anything like this for just those reason. However, having such factors be a part of the game will certainly make it more immersive, lead to enhanced role-playing, and help to add complexity to players' approaches. It won't be all about killing monsters and taking their shit, though of course these will still be primary. However, those will be means to an end: PCs are tired of living in disease-ridden squalor. That's why they are willing to face all of those terrifying foes. Because they know, in the end, the foes most likely to kill them are the ones they can't see. Some creatures are horrendous, but they are easy to understand, and the solution is usually no further away than the point of a spear or edge of a sword. Pestilence, disease, starvation, and myriad other aspects of Death, however, get short shrift in a game that's all about the war aspect. And that's a crying shame. Death must have its due, and it will take its due however it can get it. By blood or by pus, by sword, disease, or famine, Death will have its reckoning. That seems like something worth having in an Old School game, even if they are only imaginary.