user25523: Having a small comment-debate about this question. All I can find in What topics can I ask about here? is that this site is for

[questions about] biology and closely related fields, including ... general questions about biological concepts ...

The only topics that are explicitly flagged as off-topic are philosophical/ethical and personal medical questions (opinion-based and overly broad questions are also off-topic, but I don't think this question fails either of those tests).
(As a population biologist, I personally consider epidemiology to be part of "biology and closely related fields" and thus consider epidemiological questions to be on-topic, but I'm fine if that's not the consensus. For some reason :-) epidemiology seems to be a pretty hot topic at the moment ...)

user27148: In my opinion, yes, epidemiology can be on-topic because epidemiology has an interdisciplinary relationship with biology. Existing tags as a mark for what is on-topic can sometimes be misleading, but the presence and use of epidemiology suggests that at minimum some epidemiology questions are appropriate.
The sentence in the on topic page that applies most is:

Questions on interdisciplinary subjects like bioinformatics are also welcome, as long as they focus on the biological part of the subject.

Bioinformatics is called out specifically, but the phrase is "like bioinformatics"; I would argue that epidemiology is indeed like bioinformatics in relation to the rest of biology.
Like in other fields where there may be overlap with other sites, some questions in epidemiology might be better asked at CrossValidated or MedicalSciences, but that doesn't necessarily make them off-topic here.
I looked for other discussion of epidemiology on Meta, and there hasn't been all that much. One answer here I would mostly agree with, though the boundaries may be fuzzy:

Epidemiology: Biological questions? On-topic. Medical questions? Likely off-topic. Analysis questions? Off-topic. Some questions about the implications of this results? On-topic.

We have typically been averse to strictly software questions, despite the possibility that these could be broadly interpreted as related to "techniques in a biological or biochemical laboratory". I would recommend that questions about epidemiology that can be considered questions about software or programming ("how do I model these epidemiological data in R?") are not on-topic (see also Biology.SE's place now Bioinformatics.SE is in beta), but questions about concepts in epidemiology are.

25523: <p>Having a small comment-debate about <a href="https://biology.stackexchange.com/questions/90742/role-of-prevalence-in-estimating-coronavirus-mortality">this question</a>. All I can find in <a href="https://biology.stackexchange.com/help/on-topic">What topics can I ask about here?</a> is that this site is for</p>

<blockquote>
  <p>[questions about] biology and closely related fields, including ... general questions about biological concepts ...</p>
</blockquote>

<p>The only topics that are explicitly flagged as <strong>off-topic</strong> are philosophical/ethical and personal medical questions (opinion-based and overly broad questions are also off-topic, but I don't think this question fails either of those tests).</p>

<p>(As a population biologist, I personally consider epidemiology to be part of "biology and closely related fields" and thus consider epidemiological questions to be on-topic, but I'm fine if that's not the consensus. For some reason :-) epidemiology seems to be a pretty hot topic at the moment ...)</p>
 27148: <p>In my opinion, <strong>yes, epidemiology can be on-topic</strong> because epidemiology has an interdisciplinary relationship with biology. Existing tags as a mark for what is on-topic can sometimes be misleading, but the presence and use of <a href="https://biology.stackexchange.com/questions/tagged/epidemiology" class="post-tag" title="show questions tagged &#39;epidemiology&#39;" rel="tag">epidemiology</a> suggests that at minimum <em>some</em> epidemiology questions are appropriate.</p>

<p>The sentence in the <a href="https://biology.stackexchange.com/help/on-topic">on topic</a> page that applies most is:</p>

<blockquote>
  <p>Questions on interdisciplinary subjects like bioinformatics are also welcome, as long as they focus on the biological part of the subject.</p>
</blockquote>

<p>Bioinformatics is called out specifically, but the phrase is "<em>like</em> bioinformatics"; I would argue that epidemiology is indeed <em>like</em> bioinformatics in relation to the rest of biology.</p>

<p>Like in other fields where there may be overlap with other sites, some questions in epidemiology might be better asked at CrossValidated or MedicalSciences, but that doesn't necessarily make them off-topic here.</p>

<p>I looked for other discussion of epidemiology on Meta, and there hasn't been all that much. <a href="https://biology.meta.stackexchange.com/a/34/27148">One answer here</a> I would mostly agree with, though the boundaries may be fuzzy:</p>

<blockquote>
  <p>Epidemiology: Biological questions? On-topic. Medical questions? Likely off-topic. Analysis questions? Off-topic. Some questions about the implications of this results? On-topic.</p>
</blockquote>

<p>We have typically been averse to strictly software questions, despite the possibility that these could be broadly interpreted as related to "<em>techniques in a biological or biochemical laboratory</em>". I would recommend that questions about epidemiology that can be considered questions about software or programming ("how do I model these epidemiological data in R?") are not on-topic (see also <a href="https://biology.meta.stackexchange.com/questions/3705/biology-ses-place-now-bioinformatics-se-is-in-beta">Biology.SE&#39;s place now Bioinformatics.SE is in beta</a>), but questions about concepts in epidemiology are.</p>
