Modes of Transmission

It's important to understand some basic information about how HIV gets into the cells of the body and replicates, and then is transmitted to other people.
There are a number of steps in the infection process:
Biological mechanism for infection Transmission Symptoms of infection Criteria for AIDS

Biological mechanism for infection
Infection takes place when the human immunodeficiency virus (HIV) enters the body and goes through a number of steps:

1. Binding and fusion: The virus binds with receptors (CD4 and one additional) on the surface of T-lymphocytes and then fuses with the host cell, releasing its RNA into the host cell.

2. Reverse transcription: An HIV enzyme (reverse transcriptase) uses the single-stranded HIV RNA to assemble a double-stranded HIV DNA molecule.

3. Integration: The HIV DNA enters the host cell's DNA, where an HIV enzyme (integrase) “hides” the HIV DNA within the host cell's own DNA. This now-integrated DNA is called a provirus and may remain inactive for several years, producing few or no new copies of HIV.

4. Transcription: When the host cells receives a signal to activate, the provirus uses an enzyme in the host called "RNA polymerase" to create copies of the HIV genomic material, as well as shorter strands of RNA called "messenger RNA (mRNA)." The mRNA is used as a blueprint to make long chains of HIV proteins.

5. Assembly: An HIV enzyme, called protease, cuts the long chains of HIV proteins into smaller individual proteins. These small proteins come together with copies of HIV's RNA genetic material, to create a new viral particle.

6. Budding: The new virus pushes out, or buds, from the host cell. During budding, the new viruses take part of the cell's outer envelope, which acts as a covering, and is studded with protein/sugar combinations, called HIV glycoproteins. These HIV glycoproteins are necessary for the virus to bind CD4 and co-receptors on other host cells. The new copies of HIV can then move on and infect other cells.
This process destroys the CD4T cells, making the CD4 count go down, and thus weakens the immune system of the body, while it increases the HIV viral load of the body.