Alcohol and drugs can alter people's judgment. They may take risks that might expose them to HIV that they would not take when sober.
Sharing needles to inject drugs (such as heroin, speed, or anabolic steroids) is VERY dangerous and can easily spread HIV (and other serious diseases) from one person to another.
Some people who are addicted to drugs may trade sex for drugs or money to get more drugs. This may put them at greater risk of HIV, especially if they do not always practice safe sex.