Interesting note that explain how viral cause depression

How Viral Pathogens Can Cause Depression
Persistent viruses and pathogens are probably major causal factors in many of cases of depression.

Many common persistent viruses can cause depression, such as Epstein-Barr virus, coxsackie B virus, human herpes 6 virus, cytomegalovirus, influenza A, West Nile virus, as well as many other pathogens.

One mechanism that explains how such viruses cause depression is through the effects interferon-alpha: Chronic infection with any of these viruses can lead to raised interferon-alpha levels (interferon-alpha is secreted by cells of the the immune system as it tries to control the virus), and it is now known that interferon-alpha can significantly affect the serotonin system.

However, this is not the only way that a persistent viral infection can cause depression. Certain types of depression originate in the hypothalamus, which can be dysregulated by viruses such as coxsackievirus B, which have a particular affinity for, and disruptive action on, the hypothalamus.

And another very powerful (but indirect) mechanism through which persistent viruses can cause depression is by raising glutamate and quinolinic acid levels in the brain (high glutamate levels are linked to depression). The excess glutamate/quinolinic acid in this case comes from the activated microglia cells (microglia are specialized macrophages permanently resident in the brain). When the microglia are activated in response to a persistent infection in the brain, these microglia produce lots of glutamate and quinolinic acid, which can then lead to both depression and anxiety disorders as a result of this neurochemical imbalance.
I think probably by inflammation

Depression can sometimes be caused by low levels of the adrenal hormone cortisol. Now, viruses like enterovirus often chronically infect the adrenal glands, and in doing so, can alter adrenal function, leading to low cortisol output, thereby precipitating depression. In addition to depression, a chronic sub-clinical viral infection of the adrenal glands can also cause irregular, sudden spurts of adrenal hormone output, spurts of cortisol and adrenaline for example, and transient high levels of these two hormones can cause rapid heart beats (tachycardia). This is the probable explanation of the statistically-observed association between depression and tachycardia.

IN SUMMARY: It is quite probable that many cases of depression are ultimately viral in origin, through one or more of these mechanisms. We are just beginning to uncover the complex biochemistry behind the way pathogenic infections cause mental conditions such as depression.