So, I have a nearly fully fleshed-out armour system, but I'm getting seriously stuck on one part of it. I'll explain how the system works for context, in the spoiler tags below.

[spoiler]

Locational armour:

The most important part of this system is that damage and armour are locational. The game does register the difference between an attack to the torso and an attack to the leg, or an attack to the head. It also tracks what parts of you are armoured and what parts of you are not, so you can have the best breastplate in the game and it'll do absolutely nothing if you get hit in the head. This is used to create weak points in characters, areas where it can be difficult or even impossible to provide a large amount of armour, most notably the face, neck, hands and feet, and players can intentionally target these tiny hitboxes (which is even harder than it sounds) to stand the best chance against the opponent's armour. If at all possible, this should extend to the physiological resistances as well, whatever system they end up using.

Armour layers:

This is the main reason why I'm so hesitant to use percentage resistances, even more than I would be in other games. You can layer armour in this game, with some forms of armour typed as underlays (appears under overlays), overlays (appears on top of underlays) or inlays (not visible on your sprite unless worn alone). These three layers have their strengths and weaknesses and stack as to provide the best protection when all three layers are worn together. This is part of how weak points are made, not all layers are available on all areas. The neck can't have an overlay, the hands, feet and face only get one armour layer that has the protection of an inlay layer (the weakest layer) with extra weight and penalties, and as a result these areas have a much lower limit on the DR they can provide.

Armour materials:

Not all armour has the same traits. A fiber helmet and a copper helmet may be the same shape, weight and integrity, but the copper one is MUCH stronger against slashing and piercing attacks and the fiber one is MUCH stronger against bludgeoning and puncturing ones, amongst other differences. The five types are, more or less, hide clothing, textile clothing, fiber armour, metal armour and stone armour. Hide is a really cheap and common early-game clothing material not meant to provide serious protection in combat, textile is somewhat more expensive, becomes available a bit later and is much more fragile but is extraordinarily light for the protection it gives. Fiber armour is a variant on textile using mixtures of tough fibers in a dense, heavy weave to provide the maximum protection against blunt and puncturing weapons, cold temperatures and disease. Metal armour is armour made of metal, metal is late-game and scarce, generally being unique and difficult to replace if broken, but metal is strong against slash, pierce, heat, electricity, curse and poison, and is available in lead, copper, silver and gold, which have their own advantages and disadvantages, though silver is arguably the best. Stone armour, which for the record WAS a thing, works entirely differently in regards to kinetic and elemental damage and provides mediocre resistances against physiological effects. This system is also used to create weak points, as not all pieces are available in all materials.

Damage reduction:

Finally, how the armour actually protects you. Damage reduction is how armour protects against kinetic attacks, which is upwards of 90% of the actual attacks you'll be dealing with in combat (elemental damage is mostly dealt by hazards, though some enemies may use fire and vitriol from time to time). It's as simple as it sounds, X points come off each incoming hit, enemy attacks often have a penetration score that allows them to ignore X damage reduction. However, damage has special effects that require it to deal a given portion of its damage to function, such as needing to inflict 1/2 damage or more to cause bleeding, so DR is even more effective than it looks, and even one point of damage coming off is significant in a way. This means chip damage from kinetic sources is largely useless against armour. Stone armour provides damage reduction, but very little of it and is largely dependent on plate points.

Energy absorption:

Exactly what it sounds like, this is an amount of energy damage absorbed before damage gets to you. This is distinct from damage reduction in that it is a total amount, not an amount per hit. If your fiber cuirass says HA 8 and you have a natural HA 1, that means you'll be unaffected by the first 9 points of heat damage, whether that's from one attack or nine is irrelevant. Energy absorption also prevents an equal amount from dealing body damage after it is defeated, so even if that fiber cuirass has already absorbed 9 heat damage, the next 9 will only deal health damage. Energy absorption regenerates by its full value each minute, taking a maximum of two minutes to recover if both layers are depleted. This means chip damage from elemental hazards is perfectly valid against armour. Stone armour provides energy absorption, but very little of it and is largely dependent on plate points.

Plate points:

Plate points are the main defensive property of stone armour, which is the only armour type to provide it. Plate points are much like energy absorption, only one layer and without the regeneration. Stone armour will completely absorb damage that gets through DR and EA, at the expense of its plate points. Plate points cannot be replenished through any means, and when plate points are depleted the armour is destroyed. Clearly, plate points do not apply to physiological effects.

Integrity:

Health for non-stone armour. Stone doesn't need integrity because its plate points serve the same purpose and more. Armour receives damage whenever you do, and while it does benefit from its own damage reduction it does not benefit from other sources of damage reduction, so it takes more damage than you do. Physiological effects do not damage your armour's integrity. Moving on.

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So, here's my problem: None of the above systems work for physiological effects, those being curse, disease, infection and poison. So I'm left with divisive and percentage resistances, the former doesn't work for curse at all and I really don't want to resort to the latter. I do have the percentage rules written up, I just don't like straight percentages because they are nigh impossible to balance. Anybody got an alternative? Preferably a simple alternative?

Why would armor in general protect against curse, disease, etc?

Those would likely be effected by magical items, consumables, and player internal resistances relistically speaking.

Even then, you don't have to go that route, and make them quite strong effects for a player to consider to take down tougher foes.

You didn't think much about this, huh?

Be honest with you, a reply like this puts me off from investing much in any kind of reply. I suspect I'm probably not alone. If someone takes the time to try to give feedback, even if it's not useful, what is the value of a rebuke?

The oven & mitts experiment is a near-ideal case of what you appropriately call "energy bypassing armour". The purpose of armour is, in abstract terms, to have less than the full energy from the attack hurt the protected person, or to spread out damage over time and space to reduce the maximum energy or power density to harmless levels.

But infections and poisons are not energy attacks, resisting them is an intrinsical, physiological property of people. Armour is obviously irrelevant.

The oven & mitts experiment is a near-ideal case of what you appropriately call "energy bypassing armour". The purpose of armour is, in abstract terms, to have less than the full energy from the attack hurt the protected person, or to spread out damage over time and space to reduce the maximum energy or power density to harmless levels.
But infections and poisons are not energy attacks, resisting them is an intrinsical, physiological property of people. Armour is obviously irrelevant.

Oh really? Better tell this guy:


And these guys:

Also this guy:


And all of them:


Probably also this guy:


This guy too:


And while you're at it:


Seeing as how a physical medium is somehow not effective at preventing toxins, pathogens and particulates from reaching your body, surely all these people are wasting their time wearing suits specifically to prevent such things, right? That's exactly what you said. And you'd better hurry up, because people spend a lot of money on these "irrelevant" garments, and it sure looks like these fellows never got the memo.

Or, perhaps, you're dead wrong and you should bloody well know better.

That plague doctor wears a suit of waxed fabric, an impermeable layer of material that prevents bodily fluids from soaking through, which is washed after each treatment. The pathogens in the bodily fluids cannot leave said fluids and soon die off, preventing the doctor from catching the black death themselves. Their mask covers their most important orifices and contains sweet-smelling flowers and herbs that both prevent the stench from reaching them (which is its purpose) and kills pathogens on their way in (which is a happy accident). That modern doctor's scrubs serve the same purpose, but they don't waste their time with sweet-smelling flowers and herbs since our modern masks work better, and the suit is overall much lighter, cheaper and most importantly it is sterile and disposable. That foreign aid worker is there to treat the Ebola epidemic in Africa. Like with the doctor and the plague doctor, their suit is there to prevent the bodily fluids that spread the disease from reaching their skin, where it could find wounds, open sores and orifices to infect them from.

Those DEA workers wear hazmat suits, which prevent toxic chemicals not just from being inhaled, but from touching their skin. While any material would serve the latter purpose to some extent, hazmat suits are once again cheap (the fabric, at least), completely impermeable, non-reactive and disposable (again, at least the fabric is). That firefighter's suit doesn't just protect against heat, it is also there to prevent the firefighter from breathing in or making skin contact with the smoke and toxic fumes commonly found in burning buildings. Without it, they would be exposed to topical poisons and corrosives every time they did their job, especially in modern day with all the plastics we use. The beekeeper wears their suit to prevent the bees' stingers from reaching their body. It, unlike most of the more specialized suits, is just a thick layer of cotton, no different than regular clothing except in its coverage and thickness.

Those at the end are nuclear workers at Three Mile Island. Those suits, which notably were NOT lead-lined, serve the purpose of keeping radioactive particulates from touching their body or reaching any orifices, because the greatest threat was the particulates being incorporated into their body and continuing to irradiate them in the long term. Some of these workers still got radiation poisoning, but the suits reduced the severity greatly, and were especially effective at reducing the long-term consequences of exposure.

The reality is that what you're wearing is EXTREMELY important in preventing you from being poisoned, catching a disease or being irradiated. These people all know that, and that's why their employers spent so much money outfitting them with garments to protect them. These outfits do their jobs and do them well, they are not "irrelevant".

Curse/Radiation should work similarly to how you presented Energy Absorption.

Diseases, infections, and poisons could be status effect that are inflicted. Take a dagger with poison applied to it. The poison wouldn't affect the wearer of the armour unless the damage from the dagger penetrated through the armour.

A dagger slash against metal armour wouldn't pass the poison on to the wearer, but a pierce between joints would.

Also you should be a bit less rude to people, especially when your question wasn't thoroughly explained. Curse usually denotes magic, not radiation, and the examples you present wouldn't be considered as amrour by most people.

Tbh, surprised that people still try to help beside saying "If you know so well, figure yourself out"