This answer is based on the Wikipedia page for symptoms of HIV/AIDS. Hat-tip to @pojo-guy for the idea.
Look at the illnesses that HIV victims get. - pojo-guy
The page seems to be well-referenced, but as I’m not a virologist, I’ll leave this information for you to look at, and for someone with more expertise in the field to determine its accuracy. From the very few (read: one) discussion I’ve had with virologists on HIV, this all seems to check out.
The idea behind this answer is pretty straightforward: if you’re asking what will happen to a fictional compromised immune system, well, let’s see what happens to a real compromised immune system.
According to the article, in the early stages of AIDS, an HIV victim may have:
- Muscle pain (Myalgia)
- Night sweats
- Sore throat (Pharyngitis)
- Joint pain
- Nasal congestion
- Mouth/Esophageal sores
- Mouth ulcers
- Genital ulcers
- Enlarged liver/spleen
- Weight loss
- Neurological issues
While these are all symptoms caused by very common bugs, things are much worse for people in the much later stages of AIDS. Ultimately it comes down to what viruses, bacteria, fungi, and parasites are active in the region, but ultimately literally anything can infect a person whose immune system can’t stop it, leading to victims of HIV suffering from anything from GI disorders to pulmonary disorders to neurological disorders to cancers to mushrooms growing on their faces.
So, in short, literally, any disease can attack your mages. It just depends on what’s common in the region and how compromised their immune system is. I would work on developing a spell that boosts their immune system.
As an aside, the immune system works by detecting foreign substances and killing them off. This includes cancers. In all likelihood, everyone reading this post has had cancer before; the immune system just usually deals with it. The deadly cancers are the ones that can disguise themselves well enough to hide from the immune system.
Why do I bring this up? Well, if your mages’ immune system is a ball of energy acting as a shield from the outside, what happens when the mages’ own cells begin attacking them, from the inside?