The immune system protects us from succumbing to infections. It can do this when we are first exposed to a virus, bacterium, fungus or parasite. The immune system works even better if it has been primed by a prior infection or vaccination. Sometimes the immune system becomes too active and attacks our own tissues and organs, causing auto-immune conditions. More often the problem is an impaired immune system because of a persistent viral infection or a consequence of therapy to treat certain diseases such as cancer.