user5674: An extension of this question posted earlier:
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?

user69: Some thoughts based on my beginner's level knowledge of Buddhism and biology:
The prohibition on killing is based on the premise that we should not cause suffering. For something to suffer, it needs the capacity to suffer. In modern biological terms, that means an organism needs to have a nervous system and brain.
Currently it seems biologists don't agree about whether tapeworms have something that could be considered a brain (although they do have a nervous system). Thus, if you think they don't have brains, you might decide they do not suffer.
Another way of looking at this is that if tapeworms don't have brains they are not sentient, and non-sentient organisms do not suffer.

user5693: The original poster asks:  "Where to draw the line..."
Therein is the key to the answer:  there is noplace you can draw the line, thin and bright, therefore you cannot draw it.
There is, however, an answer.  The mouse and the tapeworm situations have similarities and also disparities.  The mouse will infiltrate and foul your food, and carry disease and vermin (fleas, etc) which can harm you, but practicing adequate cleanliness and good practices will mitigate that.  The mouse may find advantage not in your pantry, but simply in the safe, warm shelter.  The tapeworm is itself a disease, which will in time kill the host, and by doing so, itself perish.  
Therein lies the difference.  
If by your action a creature dies, and also by your inaction that creature also dies, then your action is not ultimately of significance to the death.  In that case, the course which prevents your own death also, is to be preferred.

user111: Theravada Buddhist Answer.
Whichever way you spin it, killing(intentionally) is bad Karma which you will have to pay for at some point in Samsara unless it becomes defunct. You can draw the line anywhere you like, but Karma is Karma. If the tapeworm can be removed without killing, you won't break the precept. Otherwise, you can use it to becomes dispassionate about the body and develop Vipassana. Meditate on your strong desire to live. Killing it will not solve your problem. You can have worse bodily conditions if you were born as an animal.
Once there was a farmer who, after having taken his precepts from a respected monk, went to look for his buffalo that had strayed into the forest. Along the way, he was caught by a python. His first thought was to use his axe to kill the snake coiled around him. Then he remembered that he had taken his precepts from a respected monk. The thought came to him for a second time and again he refrained from killing. The third time he was prompted to kill, he threw away his axe. The snake uncoiled itself and freed him! 

user254: 
Where does one draw the line for which forms of life are ok to destroy, and which ones are not?

That (i.e. "which forms of life?") might be not the right question.
If you're describing the situation based on a premise of violence versus non-violence, then another way to look at it might be aggression versus non-aggression, and/or aggression versus self-defence.
For example Buddhists are typically forbidden to tell lies, but there's the following Q&A from "Good Question Good Answer" by Bhante Dhammika. In the chapter about the five precepts, one of the question is whether you should lie to a killer to prevent them from killing. His answer was that,

"If you were sitting in a park and a terrified man ran past you and then a few minutes later another man carrying a knife ran up to you and asked you if you had seen which way the first man had gone, would you tell him the truth or would you lie to him?"
"If I had good reason to suspect that the second man was going to do serious harm to the first I would, as an intelligent caring Buddhist, have no hesitation in lying. We said before that one of the factors determining whether a deed is good or bad is intention. The intention to save a life is many times more positive than telling a lie is negative in circumstances such as these. If lying, drinking, or even stealing meant that I saved a life I should do it. I can always make amends for breaking these, but I can never bring a life back once it is gone. However, as I said before, please do not take this as a license to break the Precepts whenever it is convenient. The Precepts should be practiced with great care and only infringed in extreme cases."

I could try to argue that the difference between the killing-the-mouse and killing-the-tapeworm scenarios isn't the "form of life" but rather the difference between aggression (against the mouse) and self-defence (against the tape-worm).
Killing the mouse is a form of aggression ("I hate you, I kill you").
Killing the tape-worm (or, more specifically, medicating your own body for its malady) is arguably a form of self-defence rather than aggression -- maybe it's that which makes the difference.
You might argue that mouse-in-house is self-defence too, however IMO that (attacking-within-house) is straying ever further from defence-of-self: perhaps you ought to be thinking, of your house, "This is not mine, this is not me, this is not my self." and therefore not trying to defend it as if it were yourself.
The fact that the mouse is in a "house" implies you're a house-holder or layperson and therefore have many social responsibilities. What if it wasn't a tapeworm in you but in your parent, child, friend, etc.?
I don't want to say that it's clever to kill even tape-worms but maybe non-aggression (don't kill mice) versus self-defence (take medicine) is a way to distinguish the two scenarios.
Another consideration is, if you kill a mouse today then what will you do tomorrow? Instead you might prefer to make your house mouse-proof, or perhaps store your food and clothing in mouse-proof containers.
Similarly, if you eliminate a tapeworm today then what will you do tomorrow? I looked in the introduction to the Vinaya to see whether I could see there any instructions about parasites: and didn't find them. A "purge" is a permitted type of medicine for what that's worth. There are however several instructions in the Vinaya: about keeping yourself clean, and about eating cooked food.

user43: The Mahayana perspective on social and ecological ethics is based on the high ideals of symbiosis, harmony, and cooperation.
In the old times there were wandering monks who did not work and lived on alms - in return they shared the nectar of Dharma. They were no parasites. And if some of them were, Buddha encouraged the householders to be selective in choosing which monks to support.
Symbiosis is defined as a relationship beneficial to both sides. If the other side starts abusing its privileges to the extent that the relationship starts endangering your health and well-being, you have the right to withdraw your support. Meaning, you can stop feeding the beneficiary and let it find another sponsor. In the case of mice and other pests, this should be as simple as keeping your house clean and locking out access to food resources. In the case of the parasite, you can go on a 21-day fast; if worse comes to worst you have the right to expel.
As Trungpa Rinpoche explained, the samsaric approach involves solving the problems brute-force. The problem is solved at a very superficial level, the level of symptoms - but the root cause remains. No matter how much poison you waste on cockroaches, if your kitchen is dirty they'll keep coming back. In contrast with that, the Enlightened approach is to look in depth and solve the underlying problem. Prevent the problem altogether.
Violence is a low-skill approach. Instead, all arranged situations have supporting conditions they depend on. To the extent that you can identify and manage the supporting conditions you can control the situation.

5674: <p>An extension of this question posted earlier:
<a href="https://buddhism.stackexchange.com/questions/10465/how-not-to-kill-the-mouse-in-my-house">How not to kill the mouse in my house?</a></p>

<p>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? </p>

<p>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?</p>
 69: <p>Some thoughts based on my beginner's level knowledge of Buddhism and biology:</p>

<p>The prohibition on killing is based on the premise that we should not cause suffering. For something to suffer, it needs the capacity to suffer. In modern biological terms, that means an organism needs to have a nervous system and brain.</p>

<p>Currently it seems biologists don't agree about whether tapeworms have something that could be considered a brain (although they do have a nervous system). Thus, if you think they don't have brains, you might decide they do not suffer.</p>

<p>Another way of looking at this is that if tapeworms don't have brains they are not sentient, and non-sentient organisms do not suffer.</p>
 5693: <p>The original poster asks:  "Where to draw the line..."</p>

<p>Therein is the key to the answer:  there is noplace you can draw the line, thin and bright, therefore you cannot draw it.</p>

<p>There is, however, an answer.  The mouse and the tapeworm situations have similarities and also disparities.  The mouse will infiltrate and foul your food, and carry disease and vermin (fleas, etc) which can harm you, but practicing adequate cleanliness and good practices will mitigate that.  The mouse may find advantage not in your pantry, but simply in the safe, warm shelter.  The tapeworm is itself a disease, which will in time kill the host, and by doing so, itself perish.  </p>

<p>Therein lies the difference.  </p>

<p>If by your action a creature dies, and also by your inaction that creature also dies, then your action is not ultimately of significance to the death.  In that case, the course which prevents your own death also, is to be preferred.</p>
 111: <p><strong>Theravada Buddhist Answer.</strong></p>

<p>Whichever way you spin it, killing(intentionally) is bad Karma which you will have to pay for at some point in Samsara unless it becomes defunct. You can draw the line anywhere you like, but Karma is Karma. If the tapeworm can be removed without killing, you won't break the precept. Otherwise, you can use it to becomes dispassionate about the body and develop Vipassana. Meditate on your strong desire to live. Killing it will not solve your problem. You can have worse bodily conditions if you were born as an animal.</p>

<p>Once there was a farmer who, after having taken his precepts from a respected monk, went to look for his buffalo that had strayed into the forest. Along the way, he was caught by a python. His first thought was to use his axe to kill the snake coiled around him. Then he remembered that he had taken his precepts from a respected monk. The thought came to him for a second time and again he refrained from killing. The third time he was prompted to kill, he threw away his axe. The snake uncoiled itself and freed him! </p>
 254: <blockquote>
<p><strong>Where does one draw the line for which forms of life are ok to destroy, and which ones are not?</strong></p>
</blockquote>
<p>That (i.e. &quot;which forms of life?&quot;) might be not the right question.</p>
<p>If you're describing the situation based on a premise of <em>violence</em> versus <em>non-violence</em>, then another way to look at it might be <em>aggression</em> versus <em>non-aggression</em>, and/or <em>aggression</em> versus <em>self-defence</em>.</p>
<p>For example Buddhists are typically forbidden to tell lies, but there's the following Q&amp;A from &quot;<em>Good Question Good Answer</em>&quot; by Bhante Dhammika. In the chapter about the five precepts, one of the question is whether you should lie to a killer to prevent them from killing. His answer was that,</p>
<blockquote>
<p>&quot;<em>If you were sitting in a park and a terrified man ran past you and then a few minutes later another man carrying a knife ran up to you and asked you if you had seen which way the first man had gone, would you tell him the truth or would you lie to him?</em>&quot;</p>
<p>&quot;If I had good reason to suspect that the second man was going to do serious harm to the first I would, as an intelligent caring Buddhist, have no hesitation in lying. We said before that one of the factors determining whether a deed is good or bad is intention. The intention to save a life is many times more positive than telling a lie is negative in circumstances such as these. If lying, drinking, or even stealing meant that I saved a life I should do it. I can always make amends for breaking these, but I can never bring a life back once it is gone. However, as I said before, please do not take this as a license to break the Precepts whenever it is convenient. The Precepts should be practiced with great care and only infringed in extreme cases.&quot;</p>
</blockquote>
<p>I could try to argue that the difference between the killing-the-mouse and killing-the-tapeworm scenarios isn't the &quot;form of life&quot; but rather the difference between aggression (against the mouse) and self-defence (against the tape-worm).</p>
<p>Killing the mouse is a form of aggression (&quot;I hate you, I kill you&quot;).
Killing the tape-worm (or, more specifically, medicating your own body for its malady) is arguably a form of self-defence rather than aggression -- maybe it's that which makes the difference.</p>
<p>You might argue that mouse-in-house is self-defence too, however IMO that (attacking-within-house) is straying ever further from defence-of-self: perhaps you ought to be thinking, of your house, &quot;<a href="http://www.accesstoinsight.org/tipitaka/mn/mn.062.than.html" rel="noreferrer">This is not mine, this is not me, this is not my self.</a>&quot; and therefore not trying to defend it as if it were yourself.</p>
<p>The fact that the mouse is in a &quot;house&quot; implies you're a house-holder or layperson and therefore have many social responsibilities. What if it wasn't a tapeworm in you but in your parent, child, friend, etc.?</p>
<p>I don't want to say that it's clever to kill even tape-worms but maybe non-aggression (don't kill mice) versus self-defence (take medicine) is a way to distinguish the two scenarios.</p>
<p>Another consideration is, if you kill a mouse today then what will you do tomorrow? Instead you might prefer to make your house mouse-proof, or perhaps store your food and clothing in mouse-proof containers.</p>
<p>Similarly, if you eliminate a tapeworm today then what will you do tomorrow? I looked in the introduction to the Vinaya to see whether I could see there any instructions about parasites: and didn't find them. A &quot;purge&quot; is a permitted type of medicine for what that's worth. There are however several instructions in the Vinaya: about keeping yourself clean, and about eating cooked food.</p>
 43: <p>The Mahayana perspective on social and ecological ethics is based on the high ideals of symbiosis, harmony, and cooperation.</p>

<p>In the old times there were wandering monks who did not work and lived on alms - in return they shared the nectar of Dharma. They were no parasites. And if some of them were, Buddha encouraged the householders to be selective in choosing which monks to support.</p>

<p>Symbiosis is defined as a relationship beneficial to both sides. <em>If the other side starts abusing its privileges to the extent that the relationship starts endangering your health and well-being, you have the right to withdraw your support.</em> Meaning, you can stop feeding the beneficiary and let it find another sponsor. In the case of mice and other pests, this should be as simple as keeping your house clean and locking out access to food resources. In the case of the parasite, you can go on a 21-day fast; if worse comes to worst you have the right to expel.</p>

<p>As Trungpa Rinpoche explained, the samsaric approach involves solving the problems brute-force. The problem is solved at a very superficial level, the level of symptoms - but the root cause remains. No matter how much poison you waste on cockroaches, if your kitchen is dirty they'll keep coming back. In contrast with that, the Enlightened approach is to look in depth and solve the underlying problem. Prevent the problem altogether.</p>

<p><strong>Violence is a low-skill approach.</strong> Instead, all arranged situations have supporting conditions they depend on. To the extent that you can identify and manage the supporting conditions you can control the situation.</p>
