Immunology is both the study of the human immune system and the field of medicine that treats diseases of the immune system. Immunity is the body's ability to resist a disease, and the immune system is a bodywide network of interacting systems: bone marrow, white blood cells, the entire lymph system and even the skin. Immunology dates back to ancient civilizations, as it has long been known that certain individuals are naturally immune to some diseases, and that survivors of certain diseases are immune to the disease they survived. Since the immune system is so broad, encompassing everything from blood cells to skin, immunology is a very broad field of study. The treatment of disorders as simple as common allergies and as complex as AIDS all fall under the immunology category.
Flu shots are an example of immunology in action. Every year, the World Health Organization (WHO) makes a highly educated guess at which, out of a large number of flu bugs, are likely to be in circulation in the coming year, and formulates the 'flu shot'. Everyone, particularly those with weakened immune systems, is encouraged to get their flu shot, which causes the body to produce its own antibodies to repel those particular flus. If the WHO guesses wrong, and we are invaded by flus for which we have not received artificially induced immunities, a lot of people will come down with the flu.
Arthritis and asthma are diseases of the immune system caused by the body's hypersensitivity to certain substances. Most of the disorders addressed by immunology are those in which the body's own immune system isn't sufficient to ward off disease, and the immune system must be supplemented with external assistance. One of the most intractable of immunology's problems today is a disease that attacks the immune system itself, AIDS, or Acquired Immunodeficiency Syndrome, caused by the Human Immunodeficiency Virus (HIV).
AIDS leaves the body susceptible to infections that a healthy patient would easily combat with his or her own antibodies. Since the immune system protects the entire body, AIDS leaves the entire body vulnerable and opportunistic infections can attack multiple body organs, severely weakening the patient. A variety of drug therapies today can strengthen the immune system and prolong the life of the AIDS patient, but there is currently no cure.