(1) Description & causes
AIDS (acquired immune deficiency syndrome) is caused by the human immunodeficiency virus (HIV). First described in the early 1980s, it is characterized by a severe immunodeficiency leading to an increased risk of opportunistic infections and the development of certain cancers.

The United Nations AIDS program (UNAIDS) reported that there were approximately 14 million AIDS-related deaths in 1998, and over 47 million people have been infected with the virus since the start of the global epidemic. There has also been a rapid increase of AIDS cases among women and children. As of 1997, there were 694 cases of people infected with HIV in Singapore.

The common forms of transmission of HIV are sexual contact (both heterosexual and homosexual), blood (through transfusions or needle sharing) and from a pregnant woman to the foetus or a mother to her baby during breastfeeding. Other, rare transmission methods include accidental needle stick injury and organ transplantation. The virus has been isolated from virtually all body fluids, including blood, semen, vaginal secretions, tears, urine, and breast milk.

HIV belongs to a group of lentiviruses (so called because they cause slow infections), which in turn comes under the family of retroviruses (viruses that contain RNA as the genetic material and are known to cause cancer in their host). There are two types of virus: HIV-1 and HIV-2. HIV-1 is the virus that causes the worldwide AIDS pandemic, and HIV-2 is prevalent in Africa.

Initial destruction of white blood cells (specifically the T helper or CD4 cells) by the virus is followed by progressive, eventual depletion of other components of the immune system leading to weakened defence against infections and the growth of cancer. The virus also disseminates to various organs, particularly the central nervous system, and can directly cause a variety of effects.

Acute (primary) HIV infection progresses over time (weeks to months) to a symptomless period (up to 10 years) before symptoms of HIV infection develop. The infection terminates with full-blown AIDS. The definition of AIDS varies from country to country but a widely used classification of HIV infection is that by the United States Centers for Disease Control (CDC), which divides persons with HIV disease into four groups, depending on the stages of their disease. AIDS represents the last stage, and has proved to be a universally fatal illness. Few patients live beyond 5 years following diagnosis although survival rates have increased with advances in treatment techniques.

(2) Signs & symptoms
AIDS is a syndrome (a combination of signs and symptoms) resulting from several mechanisms, including immunodeficiency, autoimmunity and neurological disorders. Initial HIV infection may produce no symptom and some infected individuals remain without symptom for years between the time of exposure and development of AIDS.

AIDS patients generally show the following symptoms: prolonged fatigue, swollen glands (lymph nodes), long lasting fever, chills, excessive sweating (night sweats), mouth lesions and swollen gums, and frequent diarrhoea. Opportunistic infections are prevalent. These are usually caused by micro-organisms that elicit infections that are self-limiting in individuals with normal immune systems but which are serious, persistent and recurrent in persons whose immune systems are compromised. There are also AIDS-related malignancies and there may also be dementia and wasting syndrome.

(3) Diagnostic tests and procedures
Many HIV infected persons remain without symptoms, sometimes for years, before developing AIDS. When symptoms occur, they are non-specific and may be seen with other diseases. It is usually the combination of various symptoms, which leads to suspicion of HIV disease.

Important AIDS-indicator (or AIDS defining) diseases are a combination of tumours (e.g., Kaposi’s sarcoma, lymphoma) and opportunistic infections (e.g., candida or yeast infections of the oesophagus and respiratory tract, ulcers caused by herpes simplex virus, Pneumocystis carinii associated pneumonia, toxoplasmosis of the brain, tuberculosis, cytomegalovirus retinitis). There is AIDS-related dementia plus disproportionate loss of muscle mass and fat stores (wasting syndrome).

Specific laboratory tests for confirm of HIV infection include antibody and antigen detection in blood (or saliva) by ELISA (enzyme linked immunoabsorbent assay) and the Western blot. The majority of infected persons develop antibodies within several months of infection. The polymerase chain reaction (PCR) tests for HIV genetic material has shown improved sensitivity for the presence of the virus. Positive ("reactive") results mean that the person is infected with the virus and is infectious to others but do not predict when the person will go on to develop AIDS.

The most widely used test for monitoring progress of disease is the CD4 cell count. The risk of serious opportunistic infections increases with decrease counts. Levels below 200 per millilitre signify poor prognosis. Tests which measure the amount of actively replicating virus (the HIV viral load tests) correlate with disease progression and response to antiviral treatment to the HIV.

(4) Treatment
There is currently no cure for AIDS but there are specific treatments with medication to decrease or slow the growth of the virus and postpone the onset of AIDS. Many of the opportunistic infections are difficult to treat, and prophylaxis (preventive therapy) is required indefinitely to avoid a relapse.

Antiviral drugs suppress the replication of the HIV. These include the nucleoside and nucleotide analogs (e.g., AZT, ddl, ddC, 3TC), protease inhibitors (e.g., Saquinavir), and reverse transcriptase inhibitors (e.g., nevirapine, delavirdine). Combination therapy (use of a mixture of two or more drugs) has been shown to decrease the virus load and prevent development of mutant, drug-resistant strains.

Examples of treatment (and prophylaxis) for AIDs-related opportunistic infections are the use of trimethorpim-sulfamethoxazole (for P carinii infection, clarithromycin (Mycobacterium avium complex infection), amphotericin B (cryptococcosis), acyclovir and ganciclovir (for herpes simplex virus and ganciclovir infections, respectively). Combination therapies are used for the treatment of malignancies such as Kaposi’s sarcoma (e.g. daunorubicin, vinblastine) and lymphoma (e.g., modified CHOP, M-BACOD).

Haematopoietic stimulating factors (e.g., erythropoietin) are used to treat blood cell counts associated with AIDS or as a result of antiviral therapy.

To slow AIDS wasting, a high caloric diet must be maintained, with food supplementation. Megestrol acetate is administered to increase appetite and weight gain.

Research continues in drug and alternative therapies for AIDS. Several candidate vaccines have been proposed, with a few being tested currently. Until vaccination is a reality, prevention of HIV infection can best be achieved through safe sex, screening of blood products, health education and proper infection control practices, with emphasis on the concept of universal precautions.