So red flames means they're in danger ?

I think it means they'll turn into monsters or something.

I believe its like they've been marked or perhaps 'infected' by this illness which I think is caused by the demon. So maybe the demon is leeching their life out of them for power and that's what the illness is? idk we'll have to wait and see.

Oh I thought that they became infected but only start becoming monsters when they are malicious towards someone or something.

Well she said that it's cause by a demons magic stonr and I think the one that looks burning with red fiercely means he will become like a madman soon just like those infected who suddenly attact and bite humans/animals.

We still aren't sure at this point unless someone has read the raws, it's really all just speculations.