Does spreading the bite effect end at the colony if the last player survivor dies, or does it spread to helpless survivors?
Helpless survivors are not affected by the spreading of a bite effect.
Bite effects spread to the character with the lowest influence first. Helpless survivors do not have any stats (If they did, I would assume that they would have the lowest influence out of all the survivors).
The exact wording in the rules (page 11) is as follows:
Roll the exposure die again. On a blank result, the survivor that the bite effect spread to is not killed and the bite effect stops spreading. On any other result, the survivor that the bite effect spread to is killed and the bite effect spreads again. The bite effect will continue to spread until a player either chooses option 1, rolls a blank result after choosing option 2, or there are no more survivors at the location.