Source: Bestiary 6, pg(s). 134
It is possible that plague giants used to be some other form of giant, though neither plague giants nor other giants will accept this hypothesis; the plague giants reason that when a so-called lesser giant gets sick, it never becomes a plague giant. Plague giants are immune to disease and are not so much victims as they are servants of disease.
Most plague giants worship Apollyon or other deities associated with disease. Since they spread pestilence wherever they live, plague giants are shunned by most humanoids. Sometimes, plague giants lead daemon cults consisting of diseased, ostracised humanoids.