If your question deals with humans specifically, and things like disease processes, medical conditions, psychiatry, normal human physiology, etc., it belongs under medical sciences. If it is more generalized to other organisms (i.e., a question about mammalian circulation but not specifically about human physiology, or mechanisms of genetic mutations), or unrelated to humans at all (i.e., microbiology, botany, wildlife biology, evolutionary biology), then it belongs in biology.
Try your best to figure out which forum it belongs in, and we'll move the thread if we think it fits better with the other. Sometimes a thread seems properly placed at first, but further discussion makes it a better fit with the other too, so we might move it later rather than right away. The Medical Sciences forum is still fairly new, so this will probably become easier to figure out for yourself when there are more examples. Right now, the two forums are still a bit jumbled from the changes made only a couple months ago.