Source: Bestiary 6, pg(s). 134
Plague giants resemble victims of the plague or leprosy dressed in tattered rags. The average plague giant is 24 feet tall and weigh 15000 pounds.
It's 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 reasoning that when a so-called lesser giant gets sick, it doesn't become 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.