October Novel Prep Challenge

Actually, I'm not really sure how I'm going to write this year's Nano novel. Or even if I am able to write it.

I've had no idea what to write prior to starting the prep yesterday. Now, the idea I have (the only idea I have) has a protagonist who is doing bad things, and that is what the story is about. So, she is also an antagonist because she is doing these bad things. And the antagonist is really an illness, not a person or any kind of sentient being. So I'm not sure how I will write the backstory. I know it's a way off, but I'm worrying already. Any suggestions on how to characterise an illness would be welcome.

Just thought I'd share some of the things going around in my head right now. It's quite loud in there, lol, so I wanted to get it out.