Standup comedy must have been the furthest thing from Sarah Mills’ mind as she awaited some ominous test results in 2018. The diagnosis was life-altering: stage three bowel cancer, caught just in time, if also three years late. But then the comedy kicked back in. “Even the way I got told that I have cancer was funny,” says Mills. “They took me into a little side room and said [gravely]: ‘We’ve seen something angry-looking up your bum.’” Mills had almost stopped performing standup entirely by then but found that “the jokes start to write themselves. It’s a comedic gift.” Now in remission – “not high and dry, but getting there” – she is one of several comics to bring tales of serious illness to this year’s Edinburgh fringe. Her show Badass continues a surprisingly successful tradition. Comedians have an enviable capacity for finding silver linings in unlikely places. Before Badass, Mills presented the promising Chemo Chat Show online, talking to other comics, but then a new theatrical device entered her life. Mills’ colostomy bag has become a big social media hit, resplendent in TikTok dances and celebrity photo parodies. When Tory ministers revolted in July, she bought some blond wool and “dressed my bag as Boris”. The jokes, as she says, write themselves. Demystifying stoma bags and bowel cancer is one of Mills’ main goals. The recent death of campaigner Deborah James jolted awareness, but usually “people just don’t want to talk about it,” she says. The major culprit? “Poo. It feels like the very worst taboo.” Her Edinburgh show raises further issues: NHS underfunding, benefit unfairness and her disastrously belated diagnosis. Mills had a check-up three years earlier, in her late 20s, due to blood in her stool, but “they thought I was too young, and didn’t test me for it,” she says. Not everyone could make colostomy and cancer entertaining. “It’s really galvanised me as a comedian,” she says. “I’ve got a lot more to say.” Terence Hartnett is also in that curious position, crediting serious illness – and successful treatment – with inspiring his fringe debut. The symptoms? “A testicle bigger than a tennis ball!” the New Yorker announces. “I had a rare, aggressive form of testicular cancer. My doctor said: ‘I told my wife about your case, it’s so interesting.’ And I thought: ‘That’s not a good sign.’” Previously best known for a podcast, Down by the River, in which he roamed the US by van, Hartnett was also lacking focus for his comedy, pre-diagnosis. How soon did he think of this as material? “I was taking notes in that first doctor’s appointment,” he says, “and before my first surgery, while getting my sperm frozen.” Those notes became 1 Ball Show, his debut hour at the fringe. The jokes flowed easily. “It’s difficult to name a funnier cancer than testicular – it’s just an inherently funny place in your body.” Beneficial for comedians they may be, but the appeal of these shows for audiences is intriguing, particularly to those of us who have lost loved ones to serious illness. Would Mills have watched a cancer show before doing one? “Possibly not, if I’m totally honest,” she says, but then the fringe should encourage experimentation. “Exactly. Who else is telling jokes about colonoscopies?” Strong malady material can boost your career, as the veteran standups Paul Sinha and Steve Day discovered. “I’ve just finished touring my big Parkinson’s show,” says Sinha, of Hazy Little Thing Called Love. “It’s very much my magnum opus.” You would forgive him for avoiding the subject. Fresh from TV success on The Chase and Taskmaster in 2019, Sinha was diagnosed with Parkinson’s and cancelled that year’s fringe show to take stock. The former GP is now embarking on his first full fringe since then, with One Sinha Lifetime, “the difficult second post-diagnosis” show. Rather than worry about performing, he feels that his uncertain future allowed new freedom. “I don’t know when I’m going to start stammering or stumbling through my speech,” he says. “What I’ve decided is to be a little bit more forthright in my comedy. Take fewer prisoners.” It is working: this year Sinha plays his biggest fringe room yet. “I look at Parkinson’s as just another selling point, I suppose,” he says. “Not the disease, as such – more what it tells you about your life.” Steve Day found condition-based comedy tougher, initially. After several shows themed around his deafness, he hit a nerve in 2018 with a heartfelt hour about his father’s illness. Adventures in Dementia gets a reboot this year, after his father’s death. “I was going to call it Flogging a Dead Dad,” he says, but he settled for Further Adventures in Dementia. “When I first started doing it I thought: ‘There’s nothing funny here’, but you find the jokes,” says Day, who can’t help himself. “My dad was really, really ill when I first did the show. I’d think: ‘Stay alive another week – I’ve got press in.’ But it was tough, like being beaten up every day. So I’m doing it again because I hadn’t done it from a place of stability.” These shows can feel like a sort of sweary support group, but with trickier chats after, due to the fringe’s notoriously short turnaround times. During Day’s first run, people with similar dementia stories would linger as he frantically disconnected his equipment. He couldn’t hear them. “My hearing aids filled with sweat,” he recalls. “I was flattered, it means the show came across, massive compliment, but would you please fuck off?” Month-long fringe runs are draining at the best of times, so these post-traumatic shows shouldn’t be rushed into, as Beth Vyse would attest. Currently on sabbatical from live stages, she now lectures at Liverpool’s Institute of Performing Arts, and appears in Coronation Street (“My gran didn’t care when I was in the RSC. She said: ‘When you’re in Corrie, you know you’ve made it.’”) Vyse was diagnosed with breast cancer more than a decade ago, but continued performing outrageously surreal comedy, “even the night I found out”. Only after the five-year all-clear did she create a show, As Funny As Cancer, which she performed in Edinburgh in 2016. “It took me that long to deal with,” she says. “If there are still tears or emotions, I don’t think you can tell the story.” It started badly. Her first preview was read from a script, on a stool – “the audience were in floods, so sad!” – and needed a radical rethink. “‘How could it be crazier, more my style?’ So first I had this massive boob, and came bouncing in, like a space hopper,” she says. Her dark material went in splendidly weird directions – Dolly Parton and Michael Jackson impressions, mass audience inseminations – and had wide acclaim. “The tour was huge,” she recalls. “Then I said: ‘Enough is enough,’ and stopped.” With such emotive material, timing is everything. “Do it when you’re ready,” Vyse concludes. Then again, the stage is often how standups process heavy concepts. This year Alistair Barrie is doing an angry political show, Alistaircratic, beset by news-related rewrites. But it’s a breeze compared with 2015’s No More Stage Three, about how his wife, Emily, was undergoing chemotherapy for breast cancer until just before that fringe. Far from taking offence at the onstage indiscretions, she worked on his PowerPoint presentation, did promo in Edinburgh, a month in Australia, Singapore, and beyond. “Her attitude was: ‘I’ve got cancer; I might as well make the most out of it,’” says Barrie. Flyering at the fringe was an experience, though. “Someone turned to her and said: ‘That is not a fit subject for comedy! My mother-in-law died of cancer!’ and stormed off. And Emily’s standing there, with no hair, going: ‘Fucking hell.’” Emily recovered, and Barrie now stages an annual Hertfordshire Breast Unit benefit, so those gags aren’t retired entirely. There is a pattern for who gets them, he says: “The people who’ve had cancer, I always found, have the best sense of humour about it.” The fringe newcomers have further plans, too. Hartnett wants to visit colleges: “I’m going to say: ‘You just saved your life by touching yourself!’ What a gospel to spread.” Mills, meanwhile, has a Badass sitcom optioned, and may do a live sequel: the full stoma story. “There’s obviously another show, like: ‘I pooed in a bag and it went viral,’” says the comic. “Isn’t it surprising where life takes you?” The bag stays out of sight on stage, though. Audiences aren’t ready for that, and Mills is no exhibitionist. Controlled, home-filmed videos are very different from the vagaries of live performance, and her current show has a wider message about body positivity and hidden disabilities. “I talk about my body, and how it changed,” she says, “how I still love my body.” It is an interesting irony of these shows, how facing serious illness can strengthen the onstage voice. Mills is “a lot more passionate” now, and clearly on a mission, using her wit to raise awareness – toilet humour saving lives, you might say. She does sound understandably apprehensive about “reliving the day that I got told I had cancer, with a bunch of strangers” for a month. But then that diagnosis story gets the ultimate awkward laugh, the sort of moment that is impossible to manufacture. The old adage about laughter being the best medicine? It’s certainly true when you are up there telling such uneasy jokes. Badass is at the Pleasance Courtyard; 1 Ball Show is at Just the Tonic at the Mash House; Alistaircratic is at Liquid Room Annexe; Further Adventures in Dementia is at Laughing Horse at City Cafe; One Sinha Lifetime is at the Stand’s New Town theatre. Do you have an opinion on the issues raised in this article? If you would like to submit a letter of up to 300 words to be considered for publication, email it to us at guardian.letters@theguardian.com