Note this is how to utilize them in your story and not going into the physical details how how to write every type of injury, because that second one would both stretch my medical knowledge and be way too expansive for a tumblr post.
Knowing about the malady you are afflicting your character with is important. In the most optimal case this involves talking to an actual medical professional who is well-versed on that type of injury. However, since not all of us have doctors at our beck and call, a few articles from decent sources can suffice. Typically you're going to want these pieces of info:
symptoms (description fodder)
mechanism (how could this injury be sustained?)
level of impairment (know how and to what level the injury would reasonably impair someone. This info might need more digging around than the others)
healing time (know how long it takes to heal from this injury)
treatment (potentially including symptom management)
Having this knowledge will help ground your story in at least a touch of realism. Make sure your symptoms match the injury, make sure the mechanism of the injury is sufficient and just overall makes sense for the injury, don't have someone with a broken leg strolling around like nothing happened, don't have the healing time unexplainably short or long, don't treat shock by trying to cool them off, etc.
A quick note, if you are doing a fantasy/not-real injury or illness you don't get to necessarily skip this step. In fact, imo, it can become even harder since you have to fill in these details yourself. Remember, internal consistency is key.
You want the injury to matter to the plot or character development. Does the broken leg prevent a character from joining a big battle? Does the flu present an extra challenge to going to their daughter's dance recital? Does being weak or out of the action aggravate their hero complex or self worth issues prompting growth or self-reflection? Does a captive's injuries complicate a rescue mission? Does having their dominant arm injured force a usually brunt force reliant powerhouse to be more creative in their strategies? Does the overpowered character have to rely on their friends/allies more because of a certain injury?
Use the injury to show the level of a character's commitment or mental toughness, use it as an obstacle they have to work around, use it to up the stakes, use it to force them to be vulnerable or else show how unwilling they are to being vulnerable, use it to thwart the main team's perfect plan, use it to push a situation into even more dire straits, use it as an alternative to character death, etc etc. These are just a few ideas, there are numerous different ways you can use different injuries/illnesses in stories. Really think about how it could affect or reveal things about the plot, characters, and relationships.
As always, remember writing advice is highly subjective. Find what works for you. My number one writing advice is always to not take writing advice as hard truths :) happy writing!
