Are there any examples in world history where someone having a very minor non-fatal ailment (ie, something that would keep you home in bed from work but no worse… a headache, stomachache, common cold, etc.) changed the course of history? I’m envisioning some great general missing a key battle, some world leader staying at home sick and thus avoiding being assassinated, or something of that sort.
There are probably valid answers to the OP, but it’s worth mentioning that of course historically, and speaking in general, people suffered from more ailments and died much younger.
The bar for how sick you need to be to avoid working / fighting was no doubt much higher than it is today, and in most cases not what we’d call a minor ailment.
Napoleon’s defeat at Waterloo has been attributed to his delaying his attack due to a bad case of hemorrhoids, but that has been disputed.
It’s widely reputed (but also widely disputed) that Napoleon lost the battle of Waterloo because of a seriously painful case of hemorrhoids at the time.
It has been proposed that Robert E. Lee had heart disease and that a heart attack in 1863 had an effect on his conduct of the Battle of Gettysburg.
I seem to recall reading the Lee suffered illness while confronting Grant in the Overland Campaign. This might have changed the result of some battles but not history.
William “Bull” Halsey was hospitalized for a skin disease (dermatitis, psoriasis, or shingles depending on which accounts you read) in the spring of 1942. As a result, Raymond Spruance was placed in temporary command as his carrier group and Halsey missed the Battle of Midway.
I remember being told in grade school that Napoleon lost at Waterloo because he had onions with his dinner the night before, which gave him indigestion and/or constipation (which is mentioned at the end of your cite) the next day.
Field Marshal Erwin Rommel was absent from Africa when the British started their final offensive
23 October, 1942.
Rommel left General Georg Stumme in command, but Stumme dropped dead of a heart attack or something internal, and Rommel had to return. That couldn’t be good for morale… One leader meant to be off sick, another dropping dead, and the sick one returning.
This might have had a contribution to Rommel’s defeat, if only because they had to update Rommel via wireless, and the Brits were decrypting all such messages.
Would the Kansas City Royals have beaten the Phillies in the 1980 World Series if it hadn’t been for George Brett’s hemorrhoids? We will never know. 
There was that time Bush I threw up on the Japanese Prime Minister, but it’s not like they started a war over it or anything.
In 1960, Richard Nixon developed an infected knee and was sidelined for two weeks. When he resumed campaigning he had lost weight and looked generally unhealthy. He hadn’t yet fully recovered when he appeared in the first televised debate against Kennedy. It may not be the only reason he looked bad in that debate, but compare photos of Nixon and Kennedy at the debate vs. this photo taken less than two months later.
Why does this thread remind me of Niven’s “The Return of William Proxmire?” 
Arguing that this one actually changed history is kind of a stretch, but the Count of Gondomar, the Spanish ambassador to England during the reign of James I, apparently suffered from … butt problems, and needed to sit on a special chair. Gondomar was trying to promote a marriage between James’s son, the future Charles I, and the daughter of the (Catholic) Spanish king, a match that English Protestants strongly objected to.
Opponents of the marriage scored a major propaganda coup when they somehow managed to get a play satirizing the proposed marriage, Thomas Middleton’s A Game at Chess, past the Master of the Revels (despite the fact that you really, REALLY weren’t supposed to depict living monarchs on stage). The play was a massive hit until the king returned to London and shut it down. The highlight was the public display on stage of Gondomar’s special chair. (It’s unclear whether it was the real thing or a replica, but either way, bribing the servants seems to have been involved.)
It’s likely that the marriage would have fallen through anyway, but public ridicule of Gondomar’s butt problems certainly didn’t help.
Both William Henry Harrison and Zachary Taylor died of what would today probably be considered minor non-fatal diseases. Harrison of pneumonia and Taylor of gastroenteritis.
And both Garfield and McKinley would have survived their assassinations with modern medicine. (And perhaps even competent medical care of their time – Garfield’s doctors in particular were said to be especially dirty even for the 1880s and rooting around in his guts long after the germ theory of disease.)
Anyway, though it’s not exactly what you’re asking for, I’d argue the deaths of those presidents changed history. For example, Teddy Roosevelt might not have ever become president if not for McKinley’s assassination, and if not for Teddy, then maybe FDR wouldn’t have become president either.
Roosevelt also wouldn’t have been President if Garret Hobart hadn’t died of heart disease at the age of 55.
Hobart was McKinley’s first Vice President. He was respected, well-liked, and politically skilled. McKinley would certainly have kept him for a second term if he had lived. Which means Hobart, not Roosevelt, would have become President when McKinley was assassinated sixth months into his second term.
The future Emperor Augustus had a habit of getting very sick whenever a battle needed to be fought, allowing his much more talented colleague Agrippa to step in and successfully direct the battle.
It was commented on how convenient this was (and that maybe the sickness might be some kind of allergy to being in the vicinity of thousands of people with sharp swords trying to kill him). But he was definitely prone to sickness throughout his life.
Henry VIII decided a divorce was in order when his wife, the Spanish princess, had multiple miscarriages (and one daughter, Mary). While miscarriages certainly aren’t trivial, poor fertility was a non-fatal condition that severely altered the trajectory of English history.
Not sure this is true, at least among the elites who would be in a position to take a “sick day” (and who of course dominate almost all histories).
In modern times if you feel unwell you can be pretty confident (despite what googling your symptoms might tell you) that it is most likely a minor ailment that you will get over in a couple of days, so you might be inclined to power through. In pre-modern times there was a pretty good chance it was the onset of a potentially fatal illness, so it seems like you’d be more inclined to take it seriously (if you were fortunate enough to have doctors on hand, and you could spend a couple of days in bed and still eat).
Garfield, certainly. McKinley, maybe yes, maybe not. That bullet really ripped up his insides (stomach and pancreas, and blew out a kidney). Abdominal injuries are dicey even today, especially when there’s pancreatic trauma.
Just because serious illnesses were more common, does not mean that everyday coughs and colds or headaches didn’t happen. And in a time when genuinely incapacitating infections, chronic disease and injuries from dangerous manual labor were commonplace, no doubt people had to work through the small shit. Let alone during a war.
Plus of course even if someone thought some symptom was suggestive of a more serious illness coming, what difference would it make? Our ability to do anything about things like bacterial infections has been pretty limited until relatively recently.