Yes. Physical illness is often tied into depression. For instance, there are higher rates of depression on people who also have diabetes, chronic pain, heart disease, liver disease, kidney disease, and some cancers. This can complicate treatment of both the depression and the physical illness. Also, people with heart disease may not do as well when they are depressed.
Definitely. Illness is depressing. Depression makes illness worse. This creates a downward spiral. Reversing this spiral is very difficult. Chronic disease can improve with positive attitude. Attitude can improve with acceptance of chronic disease. Do not rely on a magic bullet. Acceptance of illness and improvement in attitude is hard and takes persistent effort.
Good question. Yes! 1st off, depression is a "physical condition" being a neurobiological disorder. 2ndly depression leads to a wide array of physical symptoms called somatoform. 3rdly dep is associated with many disorders such as fibromyalgia, migraine, pain syndromes. 4thly evidence is accumulating that inflammation (such as occurs in allergy, ms, infection) is associated with causing depression.