Are there indirect effects of infection that can cause depression?
What person, when hit with the flu, has not experienced amotivation, debilitating fatigue, reduced sex drive, irritability, and anorexia? These are depression-like symptoms that are a common concomitant of infection.
A series of studies over the last decade have demonstrated that immune activation as a consequence of bacterial or viral infections can induce symptoms of depression in both humans and animals. This is thought to be due to the direct effect of pro-inflammatory cytokines, as these produce depression-like symptoms in humans and animals.
Also of interest is that when patients are being treated with cytokines (such as interferons) for diseases such as hepatitis C, they will often experience an array of significant depressive symptoms, including tearfulness, reduced sexual drive, sleep disturbance, anorexia, and psychomotor retardation.
Therefore, it is clear that infections themselves can cause mood disorders indirectly through the stimulation of the immune cascade.