Kevin/Charon works with the same group Serpo does.
He disappears right away at the beginning, and has a smirk on his face at the end of the first game, suggesting that he knew about what was going on with the virus. Also, Harlow tells you that Serpo's group was testing out the virus in remote island cities to assess the viability of using the virus as a weapon. Serpo doesn't deny this when you accuse him of it at the end of the game. This probably means Kevin was in on it, and "disappeared" because he left the ship while you're stuck on it, to report to his superiors about what he saw on Banoi. Serpo likely was in the same position, but with Palanai, although things don't quite work out well for him. How the virus might work.
The way the disease works is that it stems from kuru and has mutated thanks to the chemicals. Why then are the main characters resistant? Based on their actions they are super tough and taking into account many of the main and side quests they are also highly altruistic. Wayne was neither, thus he turned. Most of the victims would not be super tough and thus they would turn. The super named zombies are generally labelled as criminals so they would turn. Harlow and Ryder White are tough, and decent, thus they are not as bad off as say a Thug or Suicider. Because of their nature and toughness the survivors may just be able to control the infection where the holiday goers, criminals and such who are infected would not.