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Why does sharing needles increase the risk of HIV transmission?

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AIDS Scanning electron micrograph of HIV-1 virions (green) budding from a cultured lymphocyte. Multiple round bumps on the cell surface represent sites of virion assembly and budding.

Sharing needles increases the risk of HIV transmission because the virus is a bloodborne pathogen that spreads through the direct transfer of infected bodily fluids, specifically blood.12

When a needle or syringe is used by an individual living with HIV, it can become contaminated with microscopic amounts of infected blood. If that same, unsterilized equipment is then used by another person, the virus can be introduced directly into their bloodstream. Because HIV is fragile and cannot survive long outside the body, this direct, immediate transfer of contaminated blood is a highly efficient route for transmission.12

To minimize these risks, public health guidelines emphasize the importance of using sterile equipment and avoiding the sharing of needles or other sharp objects.2