Stress and depression can cause the release of cortisol, which in turn can accelerate the condition of osteoporosis. The immune system is made up of subsystems. There are two types of white blood cells, or lymphocytes, that attack disease, or fight off infection. B lymphocytes are produced in the bone marrow - they fight off bacterial infections. T lymphocytes, which are produced in the thymus and other lymphatic tissue, fight viruses.
T cells come in to attack the virus and are killed.
Due to the lower number of T cells, the body is more susceptible to other viruses. The brain can also fight off sickness through the secretion of hormones from the endocrine system. A scale is the most common, but it doesn't accommodate for muscle vs fat. Here in the US, we're obsessed with thinness, yet we have higher rates of obesity than most countries. This is likely due to our sedentary lifestyle.