How does your written voice change when you have a cold? For me, everything is foggy, then almost over-focused, then foggy again. I got sick on a Sunday evening, but that day I did a slew of errands, then came home thinking I'd put my feet up and read the paper, and instead wrote seven book reviews. Just one after the other, with no trouble at all. It felt great, and the work was solid. I was done by 4 pm, watched a movie after dinner, then noticed my throat starting to go sideways. It's like my body pushed me to take care of business so I could spend the next three days choking on cough drops in my sleep.
Stomach ailments prompt a peristaltic writing style, a flow interrupted by stuttering. At least for me. I think even ingrown toenails have a voice beyond the expected "OUCH!"
Think about when you're sick and when you're well. Write a paragraph using a healthy baseline, then rewrite from the point of view of various ailments. If you have a chronic illness, start there and imagine a different illness or side effect of medication, and also imagine what you define as "wellness," which is different for each of us.