Exposing your body to things like dirt, grime, snot, etc., is actually a healthy thing to do. By exposing your body to tiny amounts of germs like that, you're actually exposing your immune system in a fairly harmless way and boosting your immune system.
Also, by placing the germs inside your mouth, where your immune system is constantly alert and working (saliva, tonsils, etc), it's relatively safe.
May sound gross, but it's true. Still, you can get ill from this practise, so be aware. I suggest washing your hands often, at the least.