Effect on screen

I want to make it that the less health i have the more intense the blood on the screen gets. How do i make that the blood is invisible when i have full health and gets more non transparent as i lose health and fades out as i regain health. I know i need to attach a plane in front of the camera but how would i code it to do this. I searched google and found nothing.

Take a look at this question - my answer implements a simple “bloody screen health indicator”.