Qustion on Disease Penetration (Augmentation) Options

[TKDNinjaInBlack]

So as I'm reading through Augmentation, I see certain penetration values for diseases are positive, and some are negative. Here's a list:

HSV-5 (1), Tuberculosis (-3), VITAS-3 (-2), Bedlam (1), Croisade (2)

If we handle penetration just like we handle armor piercing in combat, negative AP is better and reduces armor and positive adds to it. Does this mean that our viruses with positive penetration work less against protection systems and actually add dice?

I know this should be a simple answer, but whenever an increase armor notation is used for AP, it is denoted with a plus symbol with the number (ie, +2). With all of these penetration values, there is no addition symbol and has me wondering if they truly add value or if we just forgot the negatives?