Multiple leading medical resources state that the most common risk factors for HIV include:
Unprotected sex, especially with multiple partners or anonymous partners.
Injection drug use, primarily if they’re dirty needles or you share them with others. Even a small amount of blood with HIV can be passed on to you.
Dr. Hasham notes that there are also reports of HIV transmission through the placenta and pregnancy, so if a mother is infected, she can pass it on. Along with passing it on through breast milk.