Medical care varies widely between fantasy and SF worlds, though sometimes the latter has systems just as poor. Regardless of genre, it’s too convenient for wounded or sick characters to be instantly healed, without needing convalescence. It’s like when death loses meaning because dead people are easily revived without consequence; why fear for a character who can return? A recent vampire show featured them snapping each other’s necks during disputes. This would kill a mortal, but all it did to them was render them unconscious for an hour or two. Despite this, even other vampire characters would react as if someone they loved had just been murdered and they’d never see them again. Wouldn’t they be blasé about it? The neck snapping thing was little more than an inconvenience, so why react with horror? Similarly, if an audience knows characters can return from the dead or be instantly healed without issues, there’s no drama in health problems. For this reason, fights in superhero movies have no tension because everyone’s going to be fine! Removing tension is the opposite of a smart storytelling device.
It can be hard to generalize about average life expectancy before 1900 on Earth, due to it rising and falling, but it was seldom above thirty. However, if someone made it to age twenty, they could often expect to live another thirty years, especially among the wealthy, who had better access to health care. Children are more susceptible to sickness; the percentage of those reaching adulthood could be as low as 60%. In our fantasy settings, few characters will have living grandparents, or may not have both parents still alive. Most in their twenties are probably long married (even widowed, sometimes more than once) and with a few kids, and yet we seldom see this, perhaps because it gets in the way of adventuring and our escapism. When lifespans lengthened, the marriage age naturally rose, too. We can decide how long people are living, and the effects of this, based on story needs and general understanding of population health and resources.
Magical healing, or “laying on hands,” usually means channeling a god’s power through one’s body as a vessel, to heal the wounded. This often means that a holy person communes with that god, first establishing a relationship through prayer so that he’s not a stranger when calling on the god for this favor. Whether we call them a priest or another name doesn’t matter. This can exist in either SF or fantasy but is more common in the latter; SF typically has either medicine equivalent to Earth’s today or technology far in advance of us. In settings with supernatural power, we may have supernatural wounds that require supernatural healing techniques, as they may not respond to other methods. This is one justification, not that we need one, for magical healing.
Until recently on Earth, the understanding of germs was also poor, resulting in behaviors that spread illness more easily, such as people not washing their hands well or at all. The idea of invisible germs making people sick can be met with skepticism. In the early 1900s in New York, Mary Mallon, better known as Typhoid Mary, was an example of someone being a carrier, which means she showed no symptoms but can transmit that disease. She didn’t believe she had it due to the lack of symptoms; she refused to change her habits and infected others continuously until health officials imprisoned her for life to stop her. While some of that is specific to her, we can apply such concepts to inventing pandemics. A fantasy setting without multiple plagues (or lesser outbreaks) in the past may be unrealistic. That a parent (or earlier generation) perished this way is useful character backstory.
Many of us have heard of bloodletting, which has an interesting theory we can incorporate into our world. Ancient physicians believed the body had four “humors” and that an excess or deficiency in any, or a poor mixture of them, resulted in illness. The four humors were blood, yellow bile, phlegm, and black bile, which is thought to be clotted blood (it appears black). Each humor was believed to have origins in specific body parts, and properties that resulted in certain illnesses. For example, yellow bile caused warm illnesses, and so on. Bloodletting attempted to cure someone by removing an excess humor to restore them to normal. People were sometimes made to consume a food or drink to counter a perceived imbalance in humors. These doctors were wrong, of course, but the practices lasted for over two thousand years, had no healing effects (unless coincidentally), and were sometimes harmful to patients. Not only can our invented world have such theories, but maybe we have species that actually work this way.
When medical ignorance abounds, quacks proliferate. Using wild claims of perfect cures, these disreputable people try to sell or otherwise profit from questionable and unproven medical remedies, then flee before the truth is discovered. Sometimes by luck, they actually helped, and not all quacks had malicious intent. They often claimed that exotic materials added to something like oil or a balm would help their foolish or desperate victims; if we’ve invented unique lifeforms, we can create an alternative to the “snake oil salesman” idea. These could be harmful, addictive, benign, or ease symptoms but not cure as claimed. A new, similar counterpart exists today, when conspiracy theorists claim that something proven to be beneficial, like immunization shots, are making people sick instead, in defiance of evidence to the contrary.
In SF, the exploration of new worlds can result in a flow of new discoveries, including both medical problems and solutions, and therefore, more quacks. There can also be devices, like the polygraph (lie-detector) that do not measure what they claim to. Based on the education system we’ve imagined, we should form an idea of the likelihood of such scams to succeed, but even today, many believe the polygraph works.
Fantasy worlds are usually akin to our past, meaning poorer health care and shorter lives, which pushes other milestones earlier. Today we consider it scandalous that a girl in her early teens might be expected to marry and give birth (especially when married off against her will), but it was practical because she might’ve been dead before twenty. There may also be the equivalent of medical devices to supposedly measure some aspect of a person; we have discretion to decide if they actually work or not.
SF offers a wider variety of health care quality due to lack of uniformity across the genre as to technological levels. This is especially true in planet-hopping stories, as every society on a new planet will have developed skills at different rates. Even spacecraft must get their organic medical supplies from somewhere else unless these are being synthesized. Given this wide range, how do we decide who can do what? One answer is story needs. Determine what kind of armed conflict will result, what sort of weapons exist and their damage severity and type, and then how many characters must be killed, maimed, bedridden, or healed (and to what degree) to impact our story for tension.