There is a Parasite with 1 virus counter hosted on a rezzed Enigma, and the Runner has an Ice Carver installed. If the Runner encounters Chum, does not break its subroutine, and then continues the run and encounters Enigma, is the Enigma trashed?
No. Chum's ability is a required conditional effect that creates a constant +2 strength effect. When the Runner encounters the ice, both modifiers (Chum at +2 and Ice Carver at -1) apply at the same time and the strength of Enigma would be 2.