Virus Z is a new virus infecting people. Some people get Virus Z
and develop no systems. In others, Virus X causes a disease called
Disease Z.
If you have Virus Z, you have it for life and everyone you touch
gets Virus Z, if they didn’t have it already. (They get it
immediately when you touch them.) If you get Virus Z, you will
either immediately develop Disease Z, or you will never develop
Disease Z, and those two...