Plagueborn weapons are only dropped from the Afflicted. They usually have a skin resembling real skin. For example, the Plagueborn Shield actually has an eye on the outside, and a layer of skin on part of it. Plagueborn weapons are only found in Purple or Gold and are found anywhere where there are Afflicted in Cantha.
During the Factions Beta Weekend before the release of Guild Wars: Factions, "Plagueborn" was the name of the current Afflicted. This was changed between the beta weekend and the official full release.