Imagine that a person is standing at the end of the block you live on, threatening to kill you. You're reasonably certain they can do it, too--they have the ability (physical strength or a weapon, or both), they're demonstrating intent by moving toward you. Are you allowed to defend yourself from this attack, and if so, how?
Imagine that a person is standing at the door to your home, ditto.
Imagine that a person attending one of your parties, who previously was behaving like an entertaining and welcome guest, suddenly threatens you. What then?
Imagine that someone you love whom you have specifically invited into your home does it. What then?
Now imagine that each of these people, instead of intentionally threatening you, is simply the unknowing carrier of a usually-fatal illness. They don't intend you any harm; they don't even know they're exposing you to risk. Any of your answers change?
What about if their illness causes permanent injury but is never fatal? What steps, if any, are you allowed to take to prevent them injuring you this way? How far might you go to keep them from causing you this injury? Say their illness would take your vision, or your right arm; something serious that results in life-long need for accommodation of this disability.
Now imagine all of these situations represent pregnancy; the fetus is threatening your life or health, instead of an independent adult. Does that change your answers? If so, why?