I'm actually not quite sure that would be logical. Immunity to sickness is not like being immune to fire damage. It's more like you don't get sick, because your immune system efficiently destroys the microbes, but those that have already proliferated inside you still need to be taken care of. In real life you can aquire immunity to a disease upon/during the first infection, because your immune system learns to quickly recognize the pathogens, but you do still need to heal naturally.

From a purely gameplay perspective I also think curing sickness should not come so cheaply as equipping and unequipping some item, there should be a cost to sickness prevention. E.g. you can equip an otherwise sub-optimal item while fighting greater mummies or risk getting sick and having to deal with that in some way that you can not just easily and costlessly do whenever and however often the need arises.

I would suggest that Sickness Immunity should, at the very least, provide resistance and improved healing to any pre-existing sickness the PC had already contracted before gaining that Immunity, and/or fudge the die rolls towards the most gentle and beneficial outcomes (for example having the sickness wear off more quickly and be less likely to end with the optional final, powerful, potentially lethal fit). It isn't unreasonable for a player to assume that equipping a new item of Sickness Immunity is a good choice with a Sick character. However this more subtle effect for pre-existing conditions would go some way to prevent the issue Taederias raises regarding exploitation of repeated use. Multiple minor sicknesses would still bring a character low.

I think you are mixing getting immunity at first infection and by first time of beating that infection. If you would get the immunity at the moment of infection, the infection would never have the chance to spread. The point is, that in-game, you get an immunity, which means that the disease cannot affect you in any way. Otherwise it's not an immunity.

Also, instaremoving a disease by equipping an item is no less logical than instaremoving it by quaffing a healing potion or eating a herb.

Okay yeah, that might have been poorly phrased. But the state we refer to as immunity does not suddenly set in upon exterminating the last of the pathogens, but gradually, over the course of the disease. There are points in time where the immune system has already learned to recognize it and deal with it, but you are still sick.

Having immunity alleviate the problems somewhat, e.g. hasten recovery, make the symptoms less severe, reduce the chance of a final fever attack would make sense though, and it would be in line with disease immunity basically being a strengthening of your immune system.

Potions, herbs are magical and therefore could be assumed to work via directly exterminating the microbes. Also, gameplay-wise, they are consumable, therefore none of them is an infinite source of sickness removal.

Anyway, it all boils down to how you assume the immunity to work though. Option 1: something like what I described here, that is, just like actual immunity in real life. Option 2: continuously sending out waves of magical energy exterminating any and all microbes in the vicinity. I feel the first one would be the more natural interpretation, and it is also the more interesting, I think.

I think what you describe there would mean a sickness resistance in the Adom world, not sickness immunity. Your description is very much in line with getting poison resistance, either by eating something or wearing some poison resistance granting.

The way in which it would differ from simple resistance is that it would completely prevent you from getting sick while you're immune, and only if you're sick initially upon aquiring the intrinsic would it function similar to resistance that one time.

EDIT: Actually, as a further refinement of my suggestion, I say let an acquired disease immunity lessen the symptoms of currently active sickness, and/or speed up the recovery somewhat, also let it eliminate the final fever attack completely. Afterwards, it would make it impossible to get sick, just as it currently works. Thus, say, putting on an item granting disease immunity would be a more or less sure way to cure sickness, but not instantaneous, and you would be forced to deal with the effects for a while, thus providing incentive to use other methods.