Wrong. You clearly do not understand what disease means. Any condition that impairs your normal body function is in fact a disease. And yes genetic diseases do exist, including cystic fibrosis, sickle cell anemia, porphyria, Huntington disease which are so common you cannot deny their existence.
Wrong. I wouldn't call an opportunistic pathogens "contagious" because they exist and live with you since very early in your life and live happily with you throughout your entire life until you become immuno-compromised, which is when they start to cause damage to your body.
Wrong. Your immune system attacks anything it recognizes as "foreign" and has the capacity to attack almost any existing and non-existing, nature or synthetic antigens as long as they are recognized as "foreign."
Your immune system (lymphocytes) has the capacity to attack any "self" antigens but they are killed during their development. So anything goes wrong in that process can cause these "self-aggressive" lymphocytes to survive and start attacking your own body.
Also, uncontrolled release of inflammatory cytokines also cause substantial damage to your body. Meningitis causes damage not because the bacteria (N. meningitidis) produce toxin that damages the body, but that when they die they release large amount of lipopolysaccharide (an essential component of Gram Negative bacteria) which results in your white blood cells releasing huge amount of inflammatory cytokines that eventually damage your body (think allergy).
So yes, something as amazing as your body does attack itself and they have been exploited by many types of pathogens for their advantages.
Wrong. You don't realize there is such a field as veterinary oncology do you?
In fact, the Tasmanian Devils are on the brink of extinction because they are plagued by a type of cancer known as Devil facial tumour disease. It is estimated that they will go into extinction if we cannot find a cure for them in time.