What happens when a creature under an effect gains immunity to that effect?

Question

For my first example, let's say that a creature has contracted Filth Fever, and subsequently gains immunity to disease. The source could be anything, a Periapt of Health, or perhaps they gained their third level of Paladin, granting them the Divine Health ability.

What happens to the disease?

• Is the disease removed completely and instantly? (this seems highly abuseable)

• Do they retain the disease, but its effects are suspended until they no longer benefit from the immunity, ie taking off the Periapt, or falling and losing their Paladin class features?

• Do they continue to make saves against the disease, but without suffering the negatives of failed saves, until eventually they save against the disease and shake it off? (This one makes the most sense to me)

• Do they continue to make saves against the disease and suffer its negative effects until they are cured?

Second example: Fear effects.

Lets say a Paladin and Antipaladin are in a throw-down. The Antipaladin uses intimidate to demoralize the Paladin, which, due to his Aura of Cowardice ability, allows him to succeed and makes the Paladin shaken for 4 rounds. The Paladin backs up to outside the aura, regaining his immunity, then walks back in. Is he still shaken, or is he fine now?

Answer 1

While immune to an effect your are not harmfully affected by that effect in any way.

The description of immunity below includes a mention of secondary effect, it says the creature is not harmed by and does not suffer from the effect they are immune to in any way. So it is definitely not the fourth option.

It would also be weird if temporary immunity to disease would hamper your ability to cure yourself of that disease, so I wouldn't go with option 2 either because provided you succeed on the Fortitude saves, the immunity would be hampering in that case.

I also wouldn't go with the first option on account of how powerful this could be with a party sharing a single periapt of health, or just keeping it in your back pocket while you wear your amulet of natural armor. Effects like Protection from Evil also don't work this way.

So I think the logical option is that the disease runs its course, with the usual chance of curing it, but while immune to it, it can't deal damage to you or impose any disadvantages.

Similar with the fear effect, while immune to fear the fear effect is suppressed and no new fear effects can be applied. But should the immunity disappear new fear effects can be applied and existing fear effects that have not yet expired continue to work.

Immunity (Ex or Su)

A creature with immunities takes no damage from listed sources. Immunities can also apply to afflictions, conditions, spells (based on school, level, or save type), and other effects. A creature that is immune does not suffer from these effects, or any secondary effects that are triggered due to an immune effect.

Format: Immune acid, fire, paralysis; Location: Defensive Abilities.

Now one could argue that a Paladin with immunity to a disease he is in infected with is still contagious. But usually this would be specifically mentioned like in the Antipaladin's Plague Bringer ability.

Plague Bringer (Ex)

At 3rd level, the powers of darkness make an antipaladin a beacon of corruption and disease. An antipaladin does not take any damage or take any penalty from diseases. He can still contract diseases and spread them to others, but he is otherwise immune to their effects.

Format: Immune acid, fire, paralysis; Location: Defensive Abilities.