Here's a rundown of the books I found so far. I've yet to read these and am just going on the published descriptions, so if I've mischaracterized any, feel free to correct me, and if I missed any books, please let me know in comments.
Fig by Sarah Elizabeth Schantz (schizophrenia)
Trouble is a Friend of Mine by Stephanie Tromly (mental illness)
Hold Me Like a Breath by Tiffany Schmidt (autoimmune disorder)
Positively Beautiful by Wendy Mills (genetic mutation that causes cancer)
The Forgetting by Nicole Maggi
Also: Hold Me Like a Breath (pictured above) deals with organ transplants.
Tell me in comments: which of these books do you plan to read? Do you think this trend may be reaching a peak in 2015?