Import shock, or Sudden Import Death Syndrome was a bug of the original Creatures game in which recently-imported norns would occasionally die for no apparent reason. According to slink, it is because when a norn is exported, its metabolism is stopped. Sometimes when a norn is imported, its metabolism does not get restarted, leading to the creature's death. 
It is interesting to note that slink originally intended for the later C1 norn breeds (Ron Norns and the like) to be overcome with a necromould later in life. However it seems that this was discarded in favour of a simple automatic-death gene due to import shock-related fatalities.
The only way around import shock is to back up all .exp files, possibly by using the "read-only" function. This will create an error dialogue, but not crash the game.
Import shock is probably related to the as-yet un-named condition where after a crash, reopening the world would kill about half the norns in the world, seemingly at random.