A long time ago, a virus came that wiped out most of the Earth's population. Those that survived were immune. They basically figured out a way to pass on that genetic trait to their children. They began old fashioned breeding programs to create genetically desirable kids who would be immune once the virus came back (in season three or so). That's what the snake bloody thing was.

Manticore kinda ruined it for them, because they created all their kids with the immunity. Max was special though because she was given the cure for everyone. Apparently something was going to happen in season three where she caught the equivalent of a cold and every time she sneezed, she was spreading the cure so when the comet with the disease came, everyone would be immune.

At least this is what the commentary from the dvds tell me. I honestly never cared about the breeding cult.