Masturbating alone cannot lead to HIV infection; HIV requires a source of transmission. Without a source, HIV cannot arise out of nowhere. The HIV virus mainly exists in the bodily fluids of those infected, including blood, vaginal secretions, cervical mucus, breast milk, etc. In fact, if the skin is intact, there usually aren't significant amounts of the virus on the hands. Therefore, masturbating someone else cannot result in HIV transmission. Similarly, an uninfected individual masturbating will not contract HIV, as there is no source of the virus.