Leukaemia.
Which sounds bad to start because we all know Leukaemia is fucking hell on earth and cancer sucks but actually having the opportunity to sample it was awful.
I had severe osteomyelitis in my calcaneus after a small burn on my foot got infected, I was told amputation was likely due to its severity even though I'd been going to my local doctors to have it dressed and treated twice a week, anyway, long story short I took the root of surgical and maggot debridement and then 8 weeks of intense antibiotic therapy, it worked and I have very minor long term effects from it.
However, due to the intense antibiotic therapy, two and a half weeks in, I took a major turn, nobody initially knew it was due to the antibiotics, it actually took 4 days to figure it out and for them to stop and place me on a different concoction. By the time they'd figured out what was wrong I was severely neutropenic, for those of you who don't know, because I didn't, neutropenia is lack of neutrophils which are a type of white blood cell and I also had a general lack of white blood cells.
I had a leukaemia specialist come to treat me and he explained several things, just in the course of conversation I asked if this is what Leukaemia was like because it sucked and he said that even though it was hard to quantify, I was probably not even experiencing 10% of what a Leukaemia patient would be feeling.
Tl;dr: I felt the worst I'd ever felt and it wasn't even 10% of what a Leukaemia patient has to deal with on a daily basis on top of all the other shit that they've got going on.