I love surfing Wikipedia. Some of the best stuff I've found shines light on the relationship between the immune system and the mind.
This one's funny, the brand name of an early antidepressant medication: Ambivalon. The slogans write themselves. "Who cares? Not you! Am-bivalonnnn."
Ambivalon and a couple similar drugs came about as side effects during testing of potential antihistamines. Histamines control your body's immune system; you take an antihistamine to stop an allergy attack.
The anti-depressant connection to anti-histamines seems like it could be a fluke, unrelated. But the connection is deep.
The reason we get sick more over the winter is because our immune system is holding back. Colder temperatures and shorter days cause our bodies to reserve energy in case we need it for warmth.
Another indicator the immune system will hold back is the presence of depression. People experiencing depression get sick more often and longer.
Or do people who get sick more often and longer tend to experience depression? It doesn't matter whether the chicken or the egg came first.
Earlier, I wrote mental health is physical. But physical health is mental. If you're experiencing depression, you're much less likely to exercise, which makes you more likely to get sick, which makes you more likely to get depressed.
Physical health is mental. You'll never really be at peace in your body if you can't be at peace in your mind.