HIV mainly, but not exclusively, infects a type of cell of the immune system, known as CD4+ T helper cells (so called because they ‘help’ activate other immune cells).
HIV binds to the surface of these cells which allows the viral envelope to fuse with the cell membrane and the viral RNA and reverse transcriptase to be catapulted inside the cell:
As explained in What is HIV, viral RNA is then copied into viral DNA with the help of reverse transcriptase.
Viral DNA is then transported into the cell nucleus and inserted into the host DNA. Subsequent synthesis of viral RNA and proteins allows the formation of new viruses.
These are released into the blood and go on to infect other immune cells, and a cycle of viral infection and replication then ensues.
Clinical progression to AIDS is caused by the depletion of the CD4+ T helper cells.
Individuals with an intact immune system take for granted their health without realising that their body is continuously, and unsuccessfully, being attacked by viruses, bacteria, parasites, and even by cancerous cells.
In AIDS patients, the low numbers of CD4+ T helper cells result in the inability of the body to fight these external aggressors.
As a result, patients are more susceptible to diseases in the form of opportunistic infections and rare cancers. It is illness due to these life-threatening diseases that we call AIDS.
How does HIV kill infected immune cells?
HIV itself (or its by-products) may sometimes induce the death of infected CD4+ T helper cells but infected cells are also recognised as abnormal by the immune system and destroyed as a result.
This strategy keeps viral replication at bay for many years - the asymptomatic phase during which, HIV infected individuals are not ill, may not even know they are infected but are still capable of transmitting the virus to others.
Over time, the immune system becomes progressively weaker until, by mechanisms that are not fully understood, the fine balance between HIV replication and the activity of the immune system breaks, down leading to AIDS.
One can compare the relationship between HIV and the immune system to the maintenance of a dam holding a large body of water; when a leak is repaired, sooner or later pressure builds up elsewhere and another leak arises until eventually there are too many leaks to fix and the dam breaks.