HIV can happen ONLY if the partner is infected.
Usually in promiscuous persons, everyone hides information about whom they had slept with in the recent times. So potentially anyone can be infected but now know about it.
After getting the HIV infection, it takes at times 6 months to manifest in tests. Hence in non committed relationships, those having more partners can be infected.
Anal or vaginal sex has no difference in its ability to contract infections. But anal sex being more "rough" can transmit infections more than vaginal sex.
ANY sex done must be with condoms as it protects you from infections being transmitted from one to another. These infections may be anything from Fungal infection to HPV, to warts, to herpes etc.....