I don't think there was much logic to the causes - zombies could be created by a bite much like a disease but there was a scene in a graveyard where a centuries old zombie rose from a grave (more like a curse or voodoo).

I think a lot of these types of film like to keep you guessing and filling in blanks yourself.

I find its sort of a mirror of real life a doctor saves someone's life and I'll go thank goodness we have the skills and science to save this person, someone else will say oh thank goodness I prayed and god saved him. We all look at our own views while making sense of what's around us, so a scientists likely to see zombies as a disease and natives are likely to look to voodoo. Leave it unanswered and the viewer puts his own views on it.

Yeah, it was probably left obscure on purpose. We were trying to remember what the film actually did say/indicate. Apparently not much.