One of the most common medical complaints that I see sugar gliders for is the development of abscesses. Abscesses are basically a pocket of infection that forms somewhere on the body, most commonly on the head or on the neck or in the areas above the shoulders.
Abscesses can form for any number of different reasons, but they generally relate to the introduction of infection. It may be a small cut that gets infected. It may be a puncture wound. It may be something from a contaminated or dirty environment that stabs into or cuts the skin and introduces the bacteria, where it's then able to fester and grow and form what appears as a swelling, as a bump, or as a nodule on the sugar glider.
As I mentioned, we very commonly see them on the top of the head, and in some cases they may relate in males to a scent gland that the males have on the top of their head that may become secondarily infected. If you notice that your sugar glider has a bump, has a swelling, or has what feels like fluid-filled pocket in their skin, it's very important that you have them see a veterinarian. If it's an abscess, they'll need the abscess opened, drained, and they'll need to be put on the appropriate antibiotic therapy, both to cure it and hopefully to determine a method to prevent them from developing future abscesses.