Hi Aurora,
spoilers**
AP stands for after plague. It's been around 809 years since the start of the plague. The 'others' are also immortals and include all the failed attempts at the rituals over all the Ashborne generations, including some of the loyal servant lines such as Nathaniel's ancestors. So yeah there's a lot of them.
Isaac is 300 years old and Nathaniel is around 365 and currently. There is some hints dropped about the MC and the nature of the evolved plague/ 'curse' by playing Anthony's route, if you haven't already tried it.
more HUGE spoilers implied **
The MC is revered by the Ashborne's as the "bringer of life" as their ashes as the key part of the ritual that bestow immortality (described in Isaac's route). The second evolution of the plague that led to carriers or undead zombies, coincided with development and wide spread distribution of a treatment made from subject 001 (again the MC, described in Simon's route). The MC is protected by a spirit bond with a powerful and vengeful being 'the devil' and the zombie plague is a result of the devil's curse (as described in Anthony's route.) All routes hint that the MC and all attempts to tamper with immortal protection given to them by their bond with the devil is the origin of all these corrupting events including corruption of the MC themself.