|MadSci Network: Virology|
In a patient infected by HIV who is not treated, the virus can be found in all biological fluids (semen, saliva, breast milk, blood, plasma, cerebrospinal fluid, vaginal secretions). Transmission of HIV by breast milk, blood, plasma and semen are very well documented. Transmission of HIV by saliva is much more difficult. Even if viral RNA can be found at high levels in saliva, very few infectious viral particles can be found.
Saliva contains high levels of small antimicrobial proteins (e.g. defensins) that can disrupt the membrane of many bacteria and viruses, including HIV. The risk of transmission of HIV by kissing or oral sex has been well correlated with the presence of gingivitis, herpes, ulcers and other diseases of the mouth where blood or plasma containing high levels of infectious HIV can leak.
Airborne transmission of HIV (in the form of aerosols) is theoretically possible, but it requires extremely high concentrations of virus in the aerosols. Lab workers who can be exposed to ultracentrifugated viral preparations (100-10000x more concentrated than in the blood of even the most infected individual), need to wear N95 grade masks but even then it is just by way of precaution. We know the risk is there even if it is very small but we take precautions anyway. For casual contacts with an untreated infected individual (even one with a high viral load), the risk is too low to require precautions. In a treated individual, the risk is even lower.
HIV requires specific cells (CD4+ T cells and macrophages) and a humid (liquid) environment to thrive. For it to become airborne, it would require major changes to its replicative cycle and target cells that would probably lead to an altogether different pathology. Therefore, to become airborne HIV would need to change so much that it just would not be HIV anymore. For this to happen is realistically impossible.
I hope this answers your questions.
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