The White, The Dead and The Bullets

The white cell, the deadly virus and the bullets.

Traverse in a seemingly endless blood vessel, where your task is to destroy all viruses you can see, whilst growing your immunity at the same time. Growing immunity is essential, as this gives you more health and allows you to stay alive longer. You must keep yourself alive, and reach 100% immunity. 

Beware! Your immunity gradually decreases, and if it hits 0, your body is dead, and you must restart...