Immune systems can be big drains on other body systems. I would suggest researching how to build up your immune system. It take quite a long time to truly strengthen an immune system but there are short term things that can be done. Research shows that using anti-bacterial soap with Triclosan can actually weaken the immune system. Vitamin and supplement therapy helps but has to be a long term process.

An 18 credit hour schedule sounds like an awful heavy load on a body, especially one that is compromised.

In my experience, keeping a strong immune system has benefited my brain health too, or vice versa. As I continued my nutritional supplement regimen, my immune system got much stronger. My hay fever allergies have all but disappeared. My lengths of illnesses has decrease substantially. I used to spend weeks fighting with the allergens in the air. Now, I might have a day or two of very mild symptoms. Most allergens only effect me for a few hours now.

So, there is not quick fix but getting good rest is a big part of a short term fix.

I hope you feel better. I spent Christmas day and a few after with nausea/squirrely stomach. I was miserable for those few days.

My best to you.