@Jade-the-leaf-wing, hi! I’d say go based on whether it is realistic or fictional, or in between. If you want to have it more realistic, go for the illness. If you want it more horror/fantasy, have them possessed.

If that still doesn’t help, you can choose based on what you as an author want, though I will say this; illness might require a bit more research than possession, but with illness you won’t have to worry about explaining how it happened.

Hope this helps!

@E.G., Hello and thanks for helping. I'll go with horror. Once again thank you.