Mystique- Not sick (probably), and trying desperately to keep order and not let this spark rebellion.
Steve- He's going to hold out a while, and even when he does get sick he's going to try and act like he's fine at first. He won't get so far as dying sick, but it will eventually take him out of commission. Until then he'll be trying to help, taking care of the sick and stuff under the assumption that he can't catch it, and keeping order.
Justin- Justin will get mildly ill, but his real concern will be Justine, who I want to get very sick. Justin's going to be looking for help for her, by what ever means he can. This can either be his moment of sympathy and good dadness or, if things don't work out for him he might steal shit and threaten people until they fix her.
Billy- I imagine he'll get caught up in some weirdness given that he's still being held by SHIELD. I don't know exactly what SHIELD plans to do with their prisoners, whether he'd be locked up and things are going to go nuts when there aren't enough people to keep the prisoners under control, or if they'll move them. If Jasper needs someone to threaten I don't mind volunteering Billy. Or if Wanda/Daimon wanna come scoop him up from the chaos to try and take his soul. So many possibilities. Halp.
Pepper- She's going to get sick early on. Thinks it is just the flu, she and Tony can have don't touch me you're sick banter, and then boom, she's deathly ill. Because I live to make Tony sad.
Pietro- Won't get sick. Will start to come to his own in terms of leading the people of the asteroid. Maggie will get a little sick and Pietro will panic a bit, but ultimately they will both be okay. Pietro will help drive the rebellion.
Janet- Heroing, and being confused by militant Jasper. May get mildly sick or show early symptoms, might start a fight between them, maybe he tries to tell/force her to go to a quarantine at the earliest sign?
Coulson- Or Coulson could fill that role of fighting with Jasper? Either way he'll probably be fine through most of it and then start to show symptoms late in the game. He might be the kind of guy who would just take himself to quarantine though.