It is since long known that there is a strong correlation between the human immune system and depression. Depressed people seem to be more susceptible to medical ailments. However, statistical correlations are often misleading. How is it possible to know which one is the cause and which one is the effect, if there is any causal relationship at all? Recent studies have shown that the correlation may also work the other way around, i.e., physical illness may cause depression. The good news is that treating the physical illness may alleviate depressive symptoms in otherwise difficult to treat cases. Click here to read more.
Talking about the immune system, a recent study found that a weak immune system may, oddly enough, confer evolutionary advantages. Sheep with a strong immune system where found to produce less lambs than their more fragile and short-lived counterparts. This might help explain why debilitating characteristics are able to withstand the evolutionary process.
By the way, did you know that there really seems to be a link between depression and creativity?