Other factors may be involved which contribute to or cause the body to develop disease.   Some of these factors include heredity, exposure to poisonous (toxic) substances, poor nutrition, organ failure or malfunction, and poor personal behavior and choices.   Smoking, obesity, and over consumption of alcoholic beverages would be a few examples of poor personal choices which may have immediate or long term consequences for our health.

Immunity and Disease
Immunity describes the ability of an organism to resist foreign organisms or invaders which enter its body.    The immune system is designed to protect against microscopic organisms (bacteria, viruses) and foreign substances which enter an organism from outside its body.    The immune system also protects from many cancer cells which arise within our bodies.  An antigen is any foreign substance which invades the body of an organism, while a pathogen is a living antigen (such as viruses or bacteria) which invade an organism.

Many different kinds of white blood cells exist which are able to help the body fight foreign invaders in various ways.   These various ways include:

• engulfing (eating) invaders (phagocytes are white blood cells doing this)
• producing antibodies (chemicals which destroy or neutralize antigens) (lymphocytes are the kind of white blood cells which produce antibodies)
• marking antigens for attack and killing by other white blood cells

Below are graphics of two different categories of white blood cells in action:

Phagocytic White Blood Cells in Action

Lymphocyte White Blood Cell Function

It is important to note that an antibody has a specific shape to destroy a specific antigen.

Immune System Memory and Vaccinations
When organisms are exposed to disease, they make specific antibodies which destroy that antigen during their first exposure to it.   This first exposure to a disease and our making of antibodies in reaction to this to defend ourselves is sometimes called the primary immune response.   Our immune system has a memory.   This means that if we ever are exposed to that same particular disease antigen again, our immune system has a memory and will make antibodies so rapidly in response to another exposure that we will not get the disease. Our immune memory is sometimes called the secondary immune response.

Immune System Memory

Vaccinations use dead or weakened microbes or parts of them to stimulate the primary immune response or first production of antibodies.   Using dead or weakened microbes has the advantage of not making the organism sick as they would become if they caught and recovered from a disease.    Because the vaccine has stimulated the immune system, the organism will now have a memory to subsequent exposures to that disease causing antigen.

Allergies and Auto-immune Diseases
In allergies, the body's immune system produces chemicals in response to normally harmless substances which do not trouble other individuals.   These chemicals make people with allergies feel sick.   In auto-immune diseases, the body's immune system for usually unknown reasons may attack and destroy some its own cells.    Some kinds of arthritis and degenerative diseases result from auto-immune diseases.

AIDS, Cancer, and Disease Research
Some viral diseases damage the immune system which leaves it unable to cope with many antigens and other infectious agents.   AIDS is a viral disease which destroys the ability of the immune system to produce antibodies, so the afflicted individual is unable to cope with infections and cancer cells which arise within the body.

Cancer is a group of diseases resulting from gene mutations which cause cells to divide uncontrollably.   Exposure of cells to certain chemicals and radiation appears to increase the chance of mutations and thus cancer.

Biological research is constantly ongoing to find knowledge about diagnosing, preventing, treating, controlling and curing diseases in plants and animals.   The human genome project has provided a great deal of information of the genetic basis of many diseases.