The relationship between depression and infections runs in both directions. Infectious illnesses—especially chronic illnesses like Lyme disease—certainly can help cause or worsen depression. Infections can affect depression either biologically (by effects of inflammation on the brain) or psychologically (by increasing pain or fatigue). And depression can increase the risk of infection by reducing immune response or resistance to infection. That effect of depression on infection has not been directly studied for Lyme disease, but it has been studied for many other infections.
We sometimes get caught up in trying to decide which came first or what caused what. That can lead to focusing only in infection and ignoring or not treating depression. Or it can lead to focusing on depression, giving the false impression that “The doctor thinks it’s all in my head.”
We do know that treatments for depression are effective in people with infections or other chronic medical conditions. Even if depression seemed to be caused by infection or some other medical condition, that doesn’t mean that we should ignore it.
It’s not either/or. It’s often both/and. People who live with depression are more likely to develop a range of medical conditions. And people who live with chronic medical conditions are more likely to develop depression. Doctors and patients should always remember both of those things.