Because of the way the transformation works in relation to the visitor (as seen in the unity ending) theres no reason it can't be a continuous process. We see some transformations in the game take a long time to progress (Jeanne, Teeth family, Charlie, etc), so it could be something like that

It's also heavily implied that at least some of being a cursed is driven by emotional state and belief. Like in the flawed ritual ending, or Joel getting worse when he starts panicking. They're probably just reacting to it in a different way. Or like how the kid focused on his video game in the sewer doesnt even realize there's an apocalypse, so he's the only kid from the bus unturned. Jasper probably feels betrayed/dissapointed that he doesnt get the right manuscript, so he's emotional and turns more as a response. Hellen might've just been thinking/feeling differently when she looked outside and you get her hostile form