user6: There is a rather large amount of overlap between biology and medicine, and as we get more questions that are rather far on the medical site it is time to revisit the issue and clarify where exactly we draw the line and which kind of medical questions are still on-topic here.
The one unambiguous point is that we disallow personal medical questions. This position was instated very early and has the overwhelming support of the community as far as I can see, so there is nothing to change here.
Previous discussions of the issue resulted in a rough consensus that question about the biological issues behind medicine would be on-topic, but purely medical questions would not be. This discussion happened early, and lacks a bit of detail in my opinion. So I'd like to clarify here exactly what kind of medical questions are on-topic, and which are off-topic.
My personal idea would be to draw a hard line at treatment and diagnosis questions, e.g. "A patient has symptoms x,y,z, what is a possible diagnosis" or "Patient with disease x, what is the advantage of treatment y over z" and similar questions. Questions that ask for the biological concepts behind a certain treatment would still be on-topic, but anything else related to medical treatments would be off-topic.
The rationale why I think we are not the right place for this kind of question is in part that we're a biology site and probably not the best place to get good answers. There is certainly overlap between biology and medicine, but biologists are in most cases not qualified to answer medical treatment questions. The other part is that for me there are significant ethical issues connected to medicine. If I get a bad answer here about biology, I might mess up my experiments and waste my time and some money, but for medical questions the consequences might be far more severe. Of course a doctor shouldn't get advice from random people on the internet, but a Q&A site about medicine would in my opinion need to adapt their rules to deal with this issue. A purely medical site could do that and e.g. have something similar to the Skeptics requirement to support answers with references, but we don't have such rules because our subject is rather different.
So the purpose of this post is first to gather the opinion of the community on this issue, but also to ask for some ideas on how to exactly define the scope and on how we should add this to our FAQ. So if you have any ideas, feel free to post them here.

user806: The possible components of medical questions I can think of are:

epidemiology
natural history (untreated progression of the disease)
biological mechanisms behind diseases (i.e. pathogenesis)
pathology (symptoms, presentation,...)
diagnosis (examination, investigations, differential)
medical treatment (treatment choice, prognosis,...)
biological mechanisms behind treatments
historical aspects of the disease and treatments

The current FAQ is mostly in line with your opinion:

questions about the biological mechanisms behind medical conditions
  [... Off-topic:]
  personal medical questions

However, I would agree to topically separate Biology.SE from medicine more clearly, as opposed to just personal medical question. From the list above, I would accept epidemiology, as well as mechanisms behind disease and treatment as biologically relevant. Natural history may be relevant to biology but I think would be likely to accompany off-topic medical questions.
I believe these slight changes to the FAQ would be sufficient:

On-topic: questions about epidemiology or the biological mechanisms behind medical conditions
Off-topic: medical questions focussing on less biological aspects of diseases, e.g. preferred treatments or symptoms


user806: This answer is to provide an alternative opinion. I am personally happy with either of the two that I have posted - let the voting show what the community thinks. Or get posting yourself.
The real reason why medical questions are off-topic on Bio.SE isn't because medicine itself is off-topic. While certain aspects of medicine have little overlap with biology, the vast majority of medical topics would be perfectly fine to discuss within a biological context. As such, shutting medicine out completely would be an unecessary limitation to the scope of this site and take away from its diversity and the interest it can generate.
The reason why medical questions are off-topic is because they can be related to human (or even animal?) health in a potentially much more dangerous way than any other SE question ever could. This guideline is thus not for the administrative purpose of defining the content on the site, but rather to protect its users from asking and answering potentially dangerous questions. Defining this as "personal medical question" is inaccurate however, as the question doesn't need to pertain to the asker personally in order to involve health risks for somebody.
Thus, the proposed FAQ could be something more like:

On-topic: questions about biological aspects of medicine
Off-topic: questions whose answer could affect a person's health


user3971: As stated above, I think we can clearly state that personal medical questions of any nature are off topic and liability.
The line that I think should be drawn, is whether or not the question is asking for a preferred course of treatment or likely prognosis.
Some examples:
[Course of treatment; C]

A person has X, Y, Z symptoms (or known disease), what would be the
best treatment for this person? =>Not allowed.
A person has X, Y, Z symptoms (or known disease), would treatment A
or B be more appropriate and why? =>Not allowed.
A person has X, Y, Z symptoms (or known disease), why does the human
body/disease/pathogen produce symptom X/Y/Z? =>Allowed.  The lead in
person with X/Y/Z symptoms is erroneous and should be edited out,
but the question "Why does the human body produce symptom X" is of a
biologically relevant mechanism question.  Specific example: "Why
does the body sometimes respond to infection with a fever?" or "Why
do bacterial infections often induce fevers?"
What causes X symptom from R disease/pathogen? =>Allowed, same as
above.
Why is treatment A used for R disease/pathogen? =>Allowed.  Again
driving at the underlying biology/pharmacology/bio-chem mechanisms
in play.  "How does treatment A interact with disease/pathogen R,"
would be another acceptable rephrasing.
How do treatments A and B differ in how they interact with
disease/pathogen R? =>Allowed, same as above.
How do treatments A, B, and C effect the body? =>Possibly allowed
with editing and clarification.  We don't want to get into what
possible symptoms could be, and any desire to list symptoms should
be prohibited.  However, I think different underlying
immunological/pharmacological differences could be on topic.  For
example, you could get into the difference in the immune response to
live attenuated vaccines vs heat-inactivated vaccines.  Also want to
look out for generally low quality questions in this section.
I'm taking a course in [medical field] and was told X, but why isn't Y? => Rarely
allowed because I don't think these are likely to yield meaningful biological 
discussion, have a very low scope of relevance, and tend to be poor questions in 
general. 

[Prognosis; P]

A person has symptom X, what is going to happen to him/her? Not
Allowed.
A person has disease R, what is going to happen to him/her? Not
Allowed.
Would doing X make a person sick? or What would happen if a person
did X? =>Not allowed, and probably a poor question.
A person has disease/pathogen R, what will happen if they undergo
treatment A/B/C? =>Not allowed
A person has disease/pathogen R, what will happen if they don't
undergo treatment? =>Not allowed.
Why does disease/pathogen R often cause X? Allowed.  I think this
again can get at the mechanism.  Some specific examples: "Why does
Ebola often lead to death?" or "Why does obesity often lead to
diabetes?"  We have to careful that these questions don't get too
broad.

I think the good rule of thumb would a "mechanism" test:  
Can this question be properly answered in terms of the underlying biological mechanisms of [medical issue]?
If the answer to that is yes, then I think the question is probably on topic.  If not then it probably isn't.  I think it is easier to conceptualize how mechanisms are relevant in certain medical fields (ie pharmacology).  But I don't think this precludes mechanistic questions on less obvious fields like surgery.
This had me reviewing one of my first answers, which I'm beginning to feel is off topic as it sits.  I think that I could go into the mechanisms of arterial retraction, blood loss effects, or physiological responses to hemorrhage, but that is not what the question asked.  Instead I think it might be an example of C1 (categorizing the examples above under C or P), which should not be allowed.  The question could be pushed into C5, but is not currently phrased as such.  I must admit that this personally makes me sad as I had liked my attempt at bringing the subject to the public.
I also want to point out that I don't think we should limit ourselves to human medicine.  If any of the above questions had been asked in veterinary setting I think they would also be off topic ("a person" replaced with "a dog").  I get a little less clear on "lower" species or plants.  I'm inclined to apply the same rules as above.  If someone wants to ask about the prognosis of a plant virus, then I'm inclined not to answer.  If someone wanted know how some insult caused X symptom in plants, then I think it would be on topic to address.
I also think there needs to be some clear thought on how medical questions are tagged.  Should they be tagged as medical?  I think broken down into specific fields is a good idea (pharmacology, infectious diseases, oncology, etc), but I don't know if they should get an additional medical tag?  That might at least help us track how these questions are trending and their closure rate.
Lastly, as previously discussed, I think that most medical-class related questions are going to be poor questions.

Bias Note:
First, I must admit that my participation in the community has been intermittent/punctuated.  Also, if you've guessed from a few of my answers, I'm coming from the medicine side of things, though with a clear focus and experience in biological research.  More specifically I've been working in Infectious Diseases for last 10 years or so.

6: <p>There is a rather large amount of overlap between biology and medicine, and as we get more questions that are rather far on the medical site it is time to revisit the issue and clarify where exactly we draw the line and which kind of medical questions are still on-topic here.</p>

<p>The one unambiguous point is that we disallow personal medical questions. This position was instated very early and has the overwhelming support of the community as far as I can see, so there is nothing to change here.</p>

<p><a href="https://biology.meta.stackexchange.com/questions/42/where-do-we-draw-the-line-for-questions-about-medicine">Previous discussions of the issue</a> resulted in a rough consensus that question about the biological issues behind medicine would be on-topic, but purely medical questions would not be. This discussion happened early, and lacks a bit of detail in my opinion. So I'd like to clarify here exactly what kind of medical questions are on-topic, and which are off-topic.</p>

<p>My personal idea would be to draw a hard line at treatment and diagnosis questions, e.g. "A patient has symptoms x,y,z, what is a possible diagnosis" or "Patient with disease x, what is the advantage of treatment y over z" and similar questions. Questions that ask for the biological concepts behind a certain treatment would still be on-topic, but anything else related to medical treatments would be off-topic.</p>

<p>The rationale why I think we are not the right place for this kind of question is in part that we're a biology site and probably not the best place to get good answers. There is certainly overlap between biology and medicine, but biologists are in most cases not qualified to answer medical treatment questions. The other part is that for me there are significant ethical issues connected to medicine. If I get a bad answer here about biology, I might mess up my experiments and waste my time and some money, but for medical questions the consequences might be far more severe. Of course a doctor shouldn't get advice from random people on the internet, but a Q&amp;A site about medicine would in my opinion need to adapt their rules to deal with this issue. A purely medical site could do that and e.g. have something similar to the Skeptics requirement to support answers with references, but we don't have such rules because our subject is rather different.</p>

<p>So the purpose of this post is first to gather the opinion of the community on this issue, but also to ask for some ideas on how to exactly define the scope and on how we should add this to our FAQ. So if you have any ideas, feel free to post them here.</p>
 806: <p>The possible components of medical questions I can think of are:</p>

<ul>
<li>epidemiology</li>
<li>natural history (untreated progression of the disease)</li>
<li>biological mechanisms behind diseases (i.e. pathogenesis)</li>
<li>pathology (symptoms, presentation,...)</li>
<li>diagnosis (examination, investigations, differential)</li>
<li>medical treatment (treatment choice, prognosis,...)</li>
<li>biological mechanisms behind treatments</li>
<li>historical aspects of the disease and treatments</li>
</ul>

<p>The current FAQ is mostly in line with your opinion:</p>

<blockquote>
  <p>questions about the biological mechanisms behind medical conditions
  [... Off-topic:]
  personal medical questions</p>
</blockquote>

<p>However, I would agree to topically separate Biology.SE from medicine more clearly, as opposed to just personal medical question. From the list above, I would accept epidemiology, as well as mechanisms behind disease and treatment as biologically relevant. Natural history may be relevant to biology but I think would be likely to accompany off-topic medical questions.</p>

<p>I believe these slight changes to the FAQ would be sufficient:</p>

<ul>
<li>On-topic: questions about epidemiology or the biological mechanisms behind medical conditions</li>
<li>Off-topic: medical questions focussing on less biological aspects of diseases, e.g. preferred treatments or symptoms</li>
</ul>
 806: <p>This answer is to provide an alternative opinion. I am personally happy with either of the two that I have posted - let the voting show what the community thinks. Or get posting yourself.</p>

<p>The real reason why medical questions are off-topic on Bio.SE isn't because medicine itself is off-topic. While certain aspects of medicine have little overlap with biology, the vast majority of medical topics would be perfectly fine to discuss within a biological context. As such, shutting medicine out completely would be an unecessary limitation to the scope of this site and take away from its diversity and the interest it can generate.</p>

<p>The reason why medical questions are off-topic is because they can be related to human (or even animal?) health in a potentially much more dangerous way than any other SE question ever could. This guideline is thus not for the administrative purpose of defining the content on the site, but rather to protect its users from asking and answering potentially dangerous questions. Defining this as "personal medical question" is inaccurate however, as the question doesn't need to pertain to the asker personally in order to involve health risks for somebody.</p>

<p>Thus, the proposed FAQ could be something more like:</p>

<ul>
<li>On-topic: questions about biological aspects of medicine</li>
<li>Off-topic: questions whose answer could affect a person's health</li>
</ul>
 3971: <p>As stated above, I think we can clearly state that personal medical questions of any nature are off topic and liability.</p>

<p><strong>The line that I think should be drawn, is whether or not the question is asking for a preferred course of treatment or likely prognosis.</strong></p>

<p>Some examples:</p>

<p>[Course of treatment; C]</p>

<ol>
<li><p>A person has X, Y, Z symptoms (or known disease), what would be the
best treatment for this person? =>Not allowed.</p></li>
<li><p>A person has X, Y, Z symptoms (or known disease), would treatment A
or B be more appropriate and why? =>Not allowed.</p></li>
<li><p>A person has X, Y, Z symptoms (or known disease), why does the human
body/disease/pathogen produce symptom X/Y/Z? =>Allowed.  The lead in
person with X/Y/Z symptoms is erroneous and should be edited out,
but the question "Why does the human body produce symptom X" is of a
biologically relevant mechanism question.  Specific example: "Why
does the body sometimes respond to infection with a fever?" or "Why
do bacterial infections often induce fevers?"</p></li>
<li><p>What causes X symptom from R disease/pathogen? =>Allowed, same as
above.</p></li>
<li><p>Why is treatment A used for R disease/pathogen? =>Allowed.  Again
driving at the underlying biology/pharmacology/bio-chem mechanisms
in play.  "How does treatment A interact with disease/pathogen R,"
would be another acceptable rephrasing.</p></li>
<li><p>How do treatments A and B differ in how they interact with
disease/pathogen R? =>Allowed, same as above.</p></li>
<li><p>How do treatments A, B, and C effect the body? =>Possibly allowed
with editing and clarification.  We don't want to get into what
possible symptoms could be, and any desire to list symptoms should
be prohibited.  However, I think different underlying
immunological/pharmacological differences could be on topic.  For
example, you could get into the difference in the immune response to
live attenuated vaccines vs heat-inactivated vaccines.  Also want to
look out for generally low quality questions in this section.</p></li>
<li><p>I'm taking a course in [medical field] and was told X, but why isn't Y? => Rarely<br>
allowed because I don't think these are likely to yield meaningful biological 
discussion, have a very low scope of relevance, and tend to be poor questions in 
general. </p></li>
</ol>

<p>[Prognosis; P]</p>

<ol>
<li><p>A person has symptom X, what is going to happen to him/her? Not
Allowed.</p></li>
<li><p>A person has disease R, what is going to happen to him/her? Not
Allowed.</p></li>
<li><p>Would doing X make a person sick? or What would happen if a person
did X? =>Not allowed, and probably a poor question.</p></li>
<li><p>A person has disease/pathogen R, what will happen if they undergo
treatment A/B/C? =>Not allowed</p></li>
<li><p>A person has disease/pathogen R, what will happen if they don't
undergo treatment? =>Not allowed.</p></li>
<li><p>Why does disease/pathogen R often cause X? Allowed.  I think this
again can get at the mechanism.  Some specific examples: "Why does
Ebola often lead to death?" or "Why does obesity often lead to
diabetes?"  We have to careful that these questions don't get too
broad.</p></li>
</ol>

<p>I think the good rule of thumb would a "mechanism" test:  </p>

<p><strong>Can this question be properly answered in terms of the underlying biological mechanisms of [medical issue]?</strong></p>

<p>If the answer to that is yes, then I think the question is probably on topic.  If not then it probably isn't.  I think it is easier to conceptualize how mechanisms are relevant in certain medical fields (ie pharmacology).  But I don't think this precludes mechanistic questions on less obvious fields like surgery.</p>

<p>This had me reviewing one of <a href="https://biology.stackexchange.com/questions/11058/how-to-close-a-severed-artery-vein/11088#11088">my first answers</a>, which I'm beginning to feel is off topic as it sits.  I think that I could go into the mechanisms of arterial retraction, blood loss effects, or physiological responses to hemorrhage, but that is not what the question asked.  Instead I think it might be an example of C1 (categorizing the examples above under C or P), which should not be allowed.  The question could be pushed into C5, but is not currently phrased as such.  I must admit that this personally makes me sad as I had liked my attempt at bringing the subject to the public.</p>

<p>I also want to point out that I don't think we should limit ourselves to human medicine.  If any of the above questions had been asked in veterinary setting I think they would also be off topic ("a person" replaced with "a dog").  I get a little less clear on "lower" species or plants.  I'm inclined to apply the same rules as above.  If someone wants to ask about the prognosis of a plant virus, then I'm inclined not to answer.  If someone wanted know how some insult caused X symptom in plants, then I think it would be on topic to address.</p>

<p>I also think there needs to be some clear thought on how medical questions are tagged.  Should they be tagged as medical?  I think broken down into specific fields is a good idea (pharmacology, infectious diseases, oncology, etc), but I don't know if they should get an additional medical tag?  That might at least help us track how these questions are trending and their closure rate.</p>

<p>Lastly, as <a href="https://biology.meta.stackexchange.com/questions/545/wild-stab-in-the-dark-questions/548#548">previously discussed</a>, I think that most medical-class related questions are going to be poor questions.</p>

<hr>

<p><strong>Bias Note:</strong></p>

<p>First, I must admit that my participation in the community has been intermittent/punctuated.  Also, if you've guessed from a few of my answers, I'm coming from the medicine side of things, though with a clear focus and experience in biological research.  More specifically I've been working in Infectious Diseases for last 10 years or so.</p>
