Question: What would you say is the biggest sign of having a un-healthy immune system?

Mark Travis answered on 18 Jun 2010:

I guess there are two ways the immune system can be considered ‘unhealthy’: if it doesn’t fight infection properly, or if it attacks something that it shouldn’t attack.

So, if you keep getting infections all the time, then this may indicate your immune system isn’t working properly (this may be a bit indirect, as there may be other underlying reasons why people get the infections).
Also, if your immune system is harming your own body (such as in diabetes when cells in your pancreas are destroyed by the immune system, or inflammatory bowel disease when the guts are attacked), your immune system can definitely be considered ‘unhealthy’. Similarly, if you get allergies to something (really serious like allergies to peanuts, or milder allergies like in hayfever) this is a sign that the immune system is not working in the way it should (as the immune system is triggered by something it shouldn’t be triggered by).