HIV causes AIDS by destroying immune cells

Human Immunodeficiency Virus (HIV) is a retrovirus that can progress, if untreated, to the acquired immunodeficiency syndrome (AIDS). The primary target of HIV are cells of the human immune system, the CD4+ T cells, which are used for viral replication. Ultimately, these cells will be destroyed, leading to immunodeficiency in the patient.

Disease progression varies between patients

In the absence of antiretroviral therapy, HIV infection progresses over time from clinical latent HIV infection to early symptomatic HIV infection and ultimately to AIDS, the most severe manifestation of HIV infection. The average time of progression from HIV infection to AIDS is about ten years, but varies between individuals. AIDS is characterized by opportunistic infections and malignant tumors that are usually unable to develop in those with a healthy immune system and which can be fatal.

Disease stage determined by number of CD4+ T cells

Disease progression is associated with a decrease in the number of CD4+ T cells and an increase in virus particles (‘viral load’). The stage of HIV infection is determined by measuring the amount of CD4+ T cells and viral load in the blood. When the number of these cells drops below a critical level, symptoms of opportunistic infections will appear (often around or below 200 CD4+ T cells per microliter (µL)). The definition of AIDS includes both laboratory and clinical findings. Treatment should be started before the patient’s CD4+ count drops below 200, but most national guidelines recommend starting treatment around CD4+ cell counts of 350. For more information, see WHO, 2006f.

ICD classification follows the pattern of co-morbidity

In the ICD-10 classification AIDS and HIV-infections are classified as follows (WHO, 2007r):

• HIV disease resulting in infectious and parasitic diseases (ICD-10 code B20);
• HIV disease resulting in malignant neoplasms (ICD-10 code B21);
• HIV disease resulting in other specified diseases (ICD-10 code B22);
• HIV disease resulting in other conditions (ICD-10 code B23);
• Unspecified HIV disease (ICD-10 code B24).