I find the quality of meditation changes when I'm very ill. Example: a few years back I was knocked down with a very bad bacterial infection in my stomach, which led to internal bleeding, anemia, and so on. I refused a blood transfusion for it. Anyway, I was tired, cold, and occasionally delirious and hypothermic, and even though I couldn't sit up straight for long or even walk kinhin-style, I found the mind clear and there was an urgent quality to shamatha. I just kept going with it. (Three weeks of intensive antibiotics, prilosec, and oatmeal turned it around.)
Meanwhile, my wife's been having a three-year health care ordeal in which she's been very much... tested. Her meditation is much stronger, more stable, and her practice is more regular than it's ever been. She found a book recently called "How to Be Sick," by someone with chronic illness who is a student of Thich Nhat Hanh (I think it's TNH)... describing ways to continue practicing meditation incapacitated. She found it very helpful. It seems to me that many popular Buddhist titles are geared toward people in this situation, such as "When Things Fall Apart."
In general, I think people who practice regularly continue to practice regularly when ill. People who get ill either practice more (more often or with more urgency) or it all goes soft on them.
Thanks to the help of generous sponsors (most of them from DharmaWheel), I'm doing a Vajra Armor (Dorje Kotrab) self-retreat this summer. May the merit be yours!