Well, after 10+ years of DnD, it's finally come up that the entire party has been bitten by werewolves. Two of the characters had been bitten previously by, unbeknown to us, werewolves. One full moon night, we found ourselves in a one-room house and those two on watch. Awaken by howling, we're un-armored and fighting wolves, which ran into the woods after the remaining 3 of us beat them down to a few inches of their lives while sustaining heavy damage ourselves.
Now, in the Core rulebook, Werewolf lycanthropy is listed on curses, which can "usually" be removed with a remove curse spell. This would be handy as we have a cleric in the party. It doesn't list a DC or anything.
In the bestiary, it says
the spell within 3 days of the infecting lycanthrope’s attack.
Alternatively, consuming a dose of wolfsbane (Pathfinder
RPG Core Rulebook 560) gives an afflicted lycanthrope a
new Fortitude save to recover from lycanthropy.
So, which is it? Two of the members want to remain werewolves, but I do not. Would a Periapt of Health (immune to disease) protect me from being afflicted?
So far, my best interpretation is that it's a curse, which is transmitted through a disease that takes 3 days to incubate. After 3 days, you are afflicted with the curse.
I'd say it's really up to how you want to interpret it. Fantasy media such as the Elder Scrolls series portray vampirism and lycanthropy as being literal diseases, which can be contracted, spread, and even cured (though this usually required a lengthy side quest...).
Others portray it as a curse, as in the magical sense, but I am not 100% sure of how werewolf lore works in Pathfinder.
If I were running it, I'd say that it should be treated as a magically-created disease, as artificial as any magical beast, though a thriving lifeform in itself. Being immune to diseases would prevent you from being afflicted - but spell resistance would not.
Are this iteration of paladins immune? In my opinion, they should be.
At 3rd level, a paladin is immune to all diseases, including supernatural and magical diseases, including mummy rot.
We recently ruled them as immune as you suggest. Since remove disease can be used in the first 3 days after a bite we view the early stages as a disease.
Since paladins (3rd level and higher) are immune to disease they are immune to Lycanthropy.
If your GM says it is a curse then no they are not immune.