The Classes
Pirates: Organized thieves and murderers. They rule over open territory, attacking traveling parties to steal supplies and forcefully "recruit" members. They may take people hostage to use as slaves back in their camps, or as they call it, their "Ports." They use a series of hand signals, colors, and symbols to identify who they are, what crew they ride with, what their role is in that crew, and what their intentions are. Pirates tend to use dirty fighting tactics (explosives are a favorite), and pretty much always get around on motorbikes.
Poachers: Glorified zombie hunters. They take pleasure in killing as many of the Diseased as possible, but they never do it for the sake of helping others. If they're killing in the name of another human, they're doing it for a high price. They are highly competitive with each other and have rankings based on number of kills, hunting technique, and collected "trophies" (items or body parts collected from the corpses). Poachers prefer to go after the Infected, but have no qualms against gunning down any innocent people who happen to stand in their way or threaten their rank. A higher rank generally means a higher pay rate when called upon by the desperate public in need of an "extermination."
Scavengers: Dumpster divers. Scavengers are generally considered to be the rats of the human race. They tend to hang out in or near junkyards, living off of the junk and leftovers from others. There are many of them, but they tend to prefer solitude, traveling alone so they don't stand out amongst humans or Diseased. Scavengers are often considered to be dimwitted by most people, but in truth they tend to be more intelligent than they seem. They're thrifty and fast, and can get in and out of places without much notice. They know how to survive on their own without having to depend on anyone but themselves, and would probably outlast all other classes of humanity in the event that the virus were to completely annihilate it. Considering them to be the roaches of the human race would be more appropriate than calling them the rats.
Marshals: The law. There is no overall Marshal organization, but rather it's a title given to those who have set up their own method of protection and justice within each human settlement. They create their own justice system, making laws and doling out punishments as they see fit. Some Marshals utilize older, more fair methods of keeping order in their settlements while others work under a system of fear-born respect. Marshals often consist of prior law or military personnel, and can be either elected into their position, or they can take it by force. Because of that, Marshals as a whole cannot be seen as either "good" or "evil." They do their jobs and keep the people in their settlements generally alive and protected against attack. Whether they are loved or hated is up to the individual, but their authority is respected either way. To go against a Marshal is to potentially put the whole settlement at risk, so there is rarely ever a challenge to their position.