So when they were travelling the commoner got infected, and he did infect the water and that made people die later, then he himself died, and the slave brought him around and mad eit spread more.
If it wasn't a simple infection but the plague they brought it in multiple places, the slave is possibly a carrier with strong resistance or not having injuries or drank the water she was still healthy.
Unfortunately the plot just tries to mess with us through shock twists halfway, and the author simply closed it in a shitty way and we have to guess, a waste of a possibly good story and our time.