Acquired - oppOSed to congenital, 
Immune Deficiency  -  indicating the weakened immune system of the patient, 
Syndrome - conglomerate of numerous signs and symptoms involving a multitude of organ systems.
AIDS is caused by a virus now called the HIV (Human Immunodeficiency Virus). Three strains have been confirmed, HIV-I, HIV-II, HIV-III. Viruses basically are extremely small (20 - 300 nanometers) parasites, which multiply inside the cells of human beings who act as their hosts. Structurally they consist of a nucleic acid core (which carries the virus genetic information) surrounded by a coat of protein. [When the virus was discovered way back in 1982 by two independent groups of researchers, it was known by other names HTLV-III and LAV  - terminology which has now been rejected].
The HIV is made up of two concentric glycoprotein spheres with a RNA (Ribonucleic Acid) core. The outer surface of the outer sphere has certain special proteins that help the virus to attach itself to the body's immune defence cells - the T4 cells. After the virus enters the blood stream, it attaches itself to the said defence cells and its outer surface fuses with that of the cell. The RNA core then enters the cell and commandeers the reproductive machinery of the cell to replicate itself many times, These new viruses burst out of the host cell - killing it in the process - and go on to infect fresh T4 Cells. All such infected T4 cells are useless for the body and thus the human defences get crippled, leaving it open to even minor infections.
Though the HIV is present in all the body fluids, namely - blood, semen, saliva, tears, breast milk, urine and vaginal secretions, evidence available till now has implicated only blood, semen and vaginal secretions in transmission. (See Box - I for infectivity risk). There still is no conclusive proof as regarding breast milk / saliva transmitting AIDS.
After the virus enters the blood stream (known as 'exposure' in medical parlance), in almost all cases a fixed sequence of events occurs (see Box-II). A virus coming in contact / or deposited on intact skin / mucous membrane - poses no danger. But even if there is a micro abrasion (often unseen by the naked eye), the risk increases dramatically.
Unlike many medical conditions where certain typical features characterise a particular disease, AIDS is completely different. AIDS by itself doesn't have any peculiar symptoms - but depending on the organ system involved it can present itself in numerous garbs. (Hence the question what are the symptoms of AIDS? is technically a wrong question).
However as a rough guide, one should keep this disease in mind (even the term AIDS is now no longer in vogue - more preferred is the term HIV) as one of the probable diagnosis if a patient presents with any of the following symptoms. Unexplained weight loss   (more than 10 % of body weight), persistent fever (more than 1 month), diarrhoea  (more than 1 month), recurrent apthous (oral) ulcers, unexplained night sweats and recurrent herpes (A viral infection). All these just make a loose framework for one to suspect HIV positivity. You must remember that the presence of these need not always indicate AIDS, and neither are these symptoms mandatory in confirmed positive HIV patients.
The diagnosis of HIV, just clinically, both in adults and children is difficult under normal circumstances because the usual signs and symptoms are non - specific and resemble many common illnesses.
Apart from the general diseases that affect any normal individual, HIV patients often are susceptible to a group of infections called Opportunistic infections.
Organ systems most frequently afflicted by opportunistic infections include the respiratory, gastrointestinal, the skin and the nervous system (See Box - III). The reason that such infections are called opportunistic infections is that, in an HIV positive case, because of lowered immunity, certain rare and specific organisms get an opportunity to cause infection not usually available in a healthy person. As a rough benchmark, if any commonly occurring disease state presents in a widespread, flared up and rapidly progressive form in addition to it, being resistant to conventional treatment, one should suspect HIV infection.
Despite universal similarities, there are certain geographical differences, in which AIDS has been presenting itself in different parts of the world.
Especially in India, it's usually a drug resistant and widespread TB and severe diarrhoea, which have been most frequently seen, in HIV positive cases. In the US, it is pneumonia (a rare variety of it), which is the most common clinical presentation. And finally in Africa severe diarrhoea is the most common symptom, which is also known as the 'Slim disease' because of the acute weight loss it causes. Though certain findings mentioned earlier can lead one to suspect HIV infection, some tests are necessary to confirm the diagnosis.
Specific tests like the ELIZA include the detection of antibodies (or chemical agents with a fixed structure, which are formed in the body against any foreign agent - in this case the HIV is called the antigen. The foreign agent can also be a bacteria or an organ) against the HIV. There are also some other rapid fluorescence tests, which are routinely done in laboratories. And finally there is the confirmatory test - the Western blot test. However a HIV test, done with three different antigens is confirmatory enough. It is extremely important to remember here, that the tests become positive only 6-12 weeks after exposure. The latest reports say that this period might be as long as 6 months also. Before that (window period) practically nothing can help us to diagnose or suspect the condition.
 
The present status Of treatment
Specific drugs are now available for the individual opportunistic infections that occur in HIV positive patients. For HIV itself only a few drugs have been cleared by the FDA (Food and Drug Administration - USA) namely AZT (Azidothymidine) and DDI (didanosine). But there are promising signs from numerous others too. What is now being promoted is a cocktail - A combination of 2-3 drugs (from 2 different groups) administered simultaneously. Then there are of course the various vaccines on which hectic research is going on. It is to be clearly noted that drugs only delay the onset of symptoms, they do not cure AIDS.
Keeping all this in mind would be worthwhile especially for AIDS, to heed the aphorism that "PREVENTION IS NOT BETTER THAN CURE. IT IS THE ONLY CURE"