Presence and quantity in body fluids

HIV may be present in many body fluids, but its presence will not necessarily constitute a risk for infection. This is because it may be present only in very low concentrations. HIV has been found at infectious levels in five principal body fluids:

• blood
• semen
• cervicovaginal secretions
• rectal secretions 
• breast milk.

While varying levels of HIV have been found in saliva, other factors within saliva appear to inhibit HIV, rendering it uninfectious or at least much less infectious. Infectious HIV has never been successfully isolated from urine or faeces in which infected blood was not present, and is next to impossible to isolate from tears.

Levels of HIV in the bodily fluids are affected by many co-factors including sexually transmitted infections, antiretroviral treatment and the stage of infection (much higher levels are typically present in newly infected than in chronically infected individuals). These issues are covered in later sections.

HIV's genetic material may be passed from one individual to another in either of two different forms. It may exist as free viral particles, outside of any cells that are present in the infectious body fluid. These free viral particles, detectable in the form of HIV RNA, are then able to bind onto cells with CD4 markers and establish infection in the new host.

The infectious body fluid may also contain HIV-infected cells, in which HIV's genetic material is already embedded in the cell nucleus as proviral DNA. In this case, the virus may be transferred to uninfected cells in the newly infected individual. 

These two forms are not mutually exclusive. In semen, for instance, HIV exists in both forms; as 'cell-free' virus particles in the nutrient carrier fluid; and as proviral DNA within lymphocytes (white blood cells) in the seminal fluid. It is not present in sperm cells themselves. The relative importance of cell-free virus versus provirus is not completely clear, although at least one study has found that cell-free virus is the main means of transmission between gay men.¹

'Viral load' and 'infectious particles' refer to different measurements:

• 'Infectious particles' refers to the number of viruses present in millilitre of a body fluid. This is assessed by a test which detects whole viruses. It is slow and laborious to conduct, and if there are very low levels of HIV present in a body fluid the test will not detect the HIV particles.
• 'Viral load' refers to the results of a test designed to amplify any identifiable genetic fragments of the virus found in a sample of a body fluid. The viral load test detects a genetic sequence of HIV which does not tend to vary between individuals. Any such genetic 'pieces' are amplified many times, increasing its levels to quantities that are more reliably detected and measured. The final result is described as the number of HIV RNA copies per millilitre. This does not exactly correspond to the number of infectious HIV particles; it is generally at least ten times greater.

It is important to note than no more than 2% of the HIV in a person’s body is in the blood at any one time; the viral load test does not measure how much HIV is in body tissues like the lymph nodes, spleen, or brain. HIV levels in lymph tissue and semen go down when blood levels go down, but not at the same time or the same rate.