In general (Kiara obviously has one wacky exception), fish that school in the wild are less stressed when they have buddies, as a larger school means less likelihood of being eaten. They are more likely (not guaranteed, but much more likely) to be stressed when alone, and we all know that stress is a huge factor in a) not getting sick and b) recovery. :)
I wouldn't say it's a wacky exception, TBH. I have a raphael catfish who grew from 4" to 9" in 7 months & a featherfin cat who went from 1" to 8-9" within a year, both fish supposedly needing schools to even survive -- yet here they are, bigger & healthier then some people who have kept them in groups. I have a 2 year old gold white cloud who lost his friends in a columnaris outbreak[I never restocked them even tho I really like them] and he's still doing great! Even my tetras don't follow the rules. Different species school when initially introduced[something a lot of people assured me wouldn't happen which was obviously wrong] and past that, as schools die and singles remain, they can still live years past that.
But again, I'm not dissing having schools of fish but having singles aren't automatic death or unhappiness for a fish, either. Especially when they have fish of their size and shape around, they'll all form one group.
2 dogs, bearded dragon in 40gB, 4 cats[ugh! xD], 2 turtles[RES/Painted sharing a tank with an oscar & jewel cichlid], 1 temporary 10gal "QT" tank, 55gal planted FW community & 15gal nano-reef ... plus a 20gL planted divided betta tank with 4 bettas that will eventually be rehomed and the tank - decommissioned.