How can we determine whether depression causes a disease, or whether the disease leads to depression?

How can we determine whether depression causes a disease, or whether the disease leads to depression? How can depression cause the disease, whether it is the disease that leads to depression? Reply diseases that can lead to depression are often difficult, chronic and or deadly. If the disease causes depression, the pain is usually persistent or a sudden lifestyle change occurs. Depression causes the disease in a different way. Like psychological stress, it can weaken the immune system (cells involved in disease and maintain health), making the person more vulnerable to colds and flu. Sometimes there is pain without a clear reason. The presence of depression can also cause the period of illness, and the symptoms are more serious, but the true nature of the effect of depression on disease, for serious illnesses, has so far not been clearly known by science.