It's unlikely that the disease will spread without player interaction, much like how Vampires won't turn random Sims without you directing them to (playing as them or asking them to turn you). Even if it does, I'm sure they have some way of overcoming that. Maybe unplayed townies only get to level 2 of it or it comes off them if they're not on the lot or something.
I feel like if there is indeed a plague from this disease, it will be player driven. Pretty sure the devs learned their lesson back in Sims 3 when, what was it, zombies or vampires?, over-ran the place lol.
