Here is an excerpt from my book, SEVEN STEPS TO INNER POWER, where I discuss a healthy mental immune system:
Now that we've been exploring what purity means as a step to inner power, you may be wondering where hate, illness, greed, lust, revenge, and all other negatives come from if your real self knows and embodies what is pure - love, health, generosity, compassion, etc. In actuality, negative concepts are not ideas at all. They are the absence of an idea, the unreal shadows of something real. When you see a shadow on the ground, you are not fooled into thinking it has substance. However, you do know there is something of real substance nearby that is casting the shadow. In other words, the shadow tells you that there is something real at hand. Can a shadow of yourself exist without you?
Here, then, is some good news - negative ideas are substantially unreal, like shadows. When you experience them, they are only pointing to a real idea you are not expressing. This means there is really nothing standing in the way of your purity. Truly, there is only one reality, and you can make it your business to express it. You can fill the void of your negatives or anyone else's by forming the positive idea.
Take greed, for instance, clearly a negative condition that is self-destructive. Greed is the absence of the knowledge that you have the power to create all that you need out of your own ideas. Greed is the absence of the knowledge that you are complete and whole. If you truly know that you can have what you think, that you can have all you need, that you are complete and whole, you do not feel greed.
Hate is the absence of love. That thing you hate so much - isn't it true that you hate it precisely because it falls so short of what you love? "I hate him" means "I love certain qualities he does not express." Because you know that hating will only reinforce and perpetuate the negative condition, you can choose to fill this negative void with loving energy that envisions and therefore helps to bring about the positive pictures. Fear is the absence of power. You can fill this void by knowing that in truth you are a powerful being who has the inner strength to overcome any obstacle.