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 disease, and the immune system is a network of interacting systems: bone marrow, white blood cells, the entire lymphatic system, and even the skin. Immunology dates back to ancient civilizations as it has long been known that certain people are immune to some diseases and that survivors of certain diseases are immune to the disease they survived. Since the immune system is so vast, ranging from blood cells to skin, immunology is a very broad field of study. Treatment of disorders as simple as common allergies and as complex as AIDS fall under the category of immunology.
Influenza vaccines are an example of immunology in action. Every year, the World Health Organization (WHO) makes a highly educated guess as to which of a large number of flu bugs are likely to be in circulation next year, and formulates the 'flu vaccine'. Everyone, especially those with weakened immune systems, is encouraged to get a 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, many people will get 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 immune system is not sufficient to prevent disease, and the immune system must be supplemented by outside assistance. One of the most intractable immunology problems today is a disease that attacks the immune system itself, AIDS or Acquired Immune Deficiency Syndrome, caused by the Human Immunodeficiency Virus (HIV).
AIDS leaves the body susceptible to infections that a healthy patient would easily fight off with their own antibodies. Since the immune system protects the entire body, AIDS leaves the entire body vulnerable and opportunistic infections can attack multiple organs in the body, severely debilitating the patient. Today, a variety of drug therapies can strengthen the immune system and prolong the life of the AIDS patient, but there is currently no cure.