Biting can be used to sync the onset. It can be a fast acting toxin that spreads by biting and act as an activator of virulence. It can be a slow acting pathogen such a T2 virus, though thinking about it another virus will not cause immediate conversion. It would require at least few hours of incubation. My reason for not picking biting as the primary and sole way of spread is because immediate symptoms will lead to early identification and containment of the disease. Though there is nothing wrong in using biting as the only mode of transmission otherwise.