The general understanding is that if a disorder is something that comes from life experience (depression, anxiety, PTSD, etc), singular alters can have those disorders without other alters having the disorder. However, if the disorder comes from birth/genetics (autism, ADHD, etc), that will affect every alter in the system. That being said, it’s important to recognize that every alter might manage that disorder differently, and the disorder might present completely differently. One alter’s ADHD symptoms might seem much more ‘extreme’ from the outside than another’s, simply due to how they feel about it, how it affects them, and how they manage it.
Similar things can apply to allergies, diseases, accommodation needs (things like glasses, wheelchairs, canes), and more.
Take, for example, someone with a mild peanut allergy. Most alters in the system barely even notice it, and many might still eat peanuts because they just get a scratchy throat and decide it’s worth it. Some decide not to because it’s not worth it. One, however, has wildly extreme allergic reactions to peanuts whenever they eat them and gets hospitalized, and they don’t know why. It must mean alters can have different allergies, right? Wrong. In fact, that alter simply has extremely intense anxiety surrounding health concerns and allergies. They may even fit the description of a hypochondriac. The second their throat became itchy, they began scratching at it, worrying, hyperventilating, and quickly panicked. Not only did their response to their symptoms and agitation of the area massively irritate it and make the symptoms far worse and inflame the allergic response, but they were also admitted to the ER partially simply because of their own panic. This could be tied to trauma or not, but either way, when combined with amnesia, it gets very confusing for a system trying to figure out why suddenly they had a massive reaction to an allergen that has never been a big deal for them in the past.