It is probably not the case that everybody is going to become a zombie when they die.
For scientific reality on the show: it is probably the case that the virus when it enters through a small cut or eyes,etc remains subdued or dormant in healthy human beings unless the load is high from an injury or bite wound.
Now it could be the case, that the virus can be caught from human contact through touching and door knobs and enter through rubbing the nose, scratching the eyes, in which case this is probably the waiting and dormant/subdued version. This might also be the case when killing zombies, splattering blood and inhaling those particles etc.
Some people will be immune , some will have stronger or weaker immunities to the more subdued version.
Yeah that is how I surmised it. I think Rick and crew may have been immune to the original virus But now it has mutated and with them constantly battling Walkers they have gotten scrapes and scratches and been covered in blood and zombie gore enough they have the virus in them.
Now when they die their Immune system shuts down and the virus reanimates the cerebral cortex (like was shown in the last few Episodes last year at the CDC.)