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AIDS

acquired immune deficiency syndrome

According to wikipedia,

Caused by infection with the human immunodeficiency virus (HIV).Following initial infection, a person may not notice any symptoms or may experience a brief period of influenza-like illness. As the infection progresses, it interferes more with the immune system, increasing the risk of common infections like tuberculosis, as well as other opportunistic infections, and tumors that rarely affect people who have working immune systems.

SCID

Severe combined immunodeficiency

According to wikipedia,

.....is a genetic disorder characterized by the disturbed development of functional T cells and B cells caused by numerous genetic mutations that result in heterogeneous clinical presentations. SCID involves defective antibody response due to either direct involvement with B lymphocytes or through improper B lymphocyte activation due to non-functional T-helper cells. Consequently, both "arms" (B cells and T cells) of the adaptive immune system are impaired due to a defect in one of several possible genes. SCID is the most severe form of primary immunodeficiencies, and there are now at least nine different known genes in which mutations lead to a form of SCID.

Both the diseases affect the immune system of the human body. So, why they are classified as different disease? What is the property that differentiates the diseases?

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    $\begingroup$ The flu and common cold both affect the respiratory system, yet we have separate words for them. Osteoarthritis and rheumatoid arthritis both affect the joints. Why do you think diseases are solely distinguished by what part or the body they affect? $\endgroup$ – YviDe Mar 20 '16 at 9:01
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    $\begingroup$ Homework questions and trivial questions about basic biological concepts are off-topic on Biology unless you have shown your attempt at an answer. For more information see our homework policy. "Homework" is interpreted to mean any academic or other assignment, test preparation, or task given in relation to a class, educational setting, or self-learning. $\endgroup$ – MattDMo Mar 20 '16 at 17:54
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AIDS -

  1. It is caused by Human Immunodeficiency Virus (HIV).

  2. HIV attacks helper T cells and, therefore, helper T cells reduce in number, i.e. individual can produce T cells, but they are destroyed by the virus.

3.HIV can be transmitted at any stage.

SCID -

  1. It is a genetic disorder.

  2. The individuals are born without T-cells and B-cells and are incompetent to produce them.

  3. This disorder is found in new born children.

A disease is characterised not by its target organ/tissue, but how it infects that organ.

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HIV (Human immunodeficiency virus)

Cause:

  • HIV is a retrovirus that infects helper T-lymphocytes
  • Reverse transcriptase allows viral DNA to be produced from its RNA code
  • After years of inactivity (Th cells continued reproducing), the virus becomes active and starts to destroy T-lymphocytes
  • This results in lower immunity, as antibody production is compromised
  • HIV is the lowering of the immunity of a certain individual, over a number of years
  • AIDS is the final stage where observable symptoms develop

Transmission:

  • Exchange of body fluids (sexual intercourse, breastfeeding, childbirth, needles, etc.)
  • There is a very small fraction of people who are immune to HIV (no CD4+T receptor)

SCID (Severe combined immune deficiency)

A severe combined immune deficiency, a rare genetic disorder in which affected children have no resistance to disease and must be kept isolated from infection from birth.

  • Genetic disorder characterized by the disturbed development of functional T cells and B cells caused by numerous genetic mutations that result in heterogeneous clinical presentations
  • SCID involves defective antibody response due to either direct involvement with B lymphocytes or through improper B lymphocyte activation due to non-functional T-helper cells
  • Consequently, both "arms" (B cells and T cells) of the adaptive immune system are impaired due to a defect in one of several possible genes

Sources:

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