Immunity is the ability of the body to resist the infecting agent. When an infectious agent enters the body, the immune system develops antibodies which can weaken or destroy the disease-producing agent or neutralize its toxins. If the body is re-introduced to the same agent at a later time, it is capable of developing antibodies at a much faster pace. As a result, the individual would likely not become sick, and immunity has developed.
Natural immunity is present when a person is immune to a disease despite not having either the disease itself nor any vaccination against it. Acquired immunity may be either active or passive. Active immunity comes from having the disease or by inoculation with antigens, such as dead organisms, weakened organisms, or toxins of organisms. The antigens introduced during vaccination produce antibodies that protect the body against the infecting agent, despite the fact that the person does not become sick.
When the immune system successfully controls an infection on its own, it becomes stronger and better able to handle future threats. Antibiotics are powerful medicines that should be given only when the immune system cannot contain a bacterial infection. Overuse of antibiotics may cause the body to breed new strains of antibiotic-resistant or more dangerous bacteria. In the long run, overuse of antibiotics weakens the immune system.
Immunosuppressive drugs used in cancer chemotherapy or to suppress rejection of organ transplants are necessary. Of greater concern is the widespread use of corticosteroids or steroid derivatives used to treat allergies, autoimmune diseases, and inflammatory conditions. Though sometimes necessary, these drugs cripple the immune response and are often misunderstood, abused, and over-prescribed.
Heightened reactivity to antigens (molecules capable of stimulating an immune response). Many different examples of hypersensitivity have been recognized in animals and humans. These are often referred to collectively as allergies, and clinically may take such forms as asthma, hives, hay fever, anaphylactic reactions to certain foods or insect venoms, some forms of eczema and kidney diseases, and skin reactions to poison ivy antigens and many other substances. See also Antigen.
Because molecules foreign to the body are often antigenic, the various forms of hypersensitivity are most commonly induced either by exposure to foreign antigens derived from microorganisms during infections, or by contact with certain noninfectious agents (some plant pollens, some drugs, and certain simple chemicals such as components of poison ivy). However, under certain circumstances, molecules of the body itself can induce an immune response. In these cases, hypersensitivity reactions can be directed against antigens of the body's own organs or tissues. Whether foreign or derived from the body itself, antigenic substances often produce little or no tissue reaction in unsensitized individuals. But once hypersensitivity develops, additional exposure to antigen can give rise to clinically obvious symptoms (hives, sneezing, runny nose), tissue damage, or even (in certain extreme cases) death.
Doctors and biologists say that it's possible that in a few generations, Earth's population will become completely immune to regular drugs and that previously cured and extinct diseases might resurrect.