If a buddhist should not kill a mouse living in their home, what justification do they have to rid themselves of a parasite such tapeworms

An extension of this question posted earlier: https://buddhism.stackexchange.com/questions/10465/how-not-to-kill-the-mouse-in-my-house Where does one draw the line for which forms of life are ok to destroy, and which ones are not? Is it just a calculation of causing the least suffering? A human has a more complex nervous system than a worm, so to reduce his suffering is paramount? I can think of many ways to spin the issue so that the human must allow himself to die, and ways to spin it so that killing living creatures is acceptable. Are there some general guidelines?