Drugs are abused in a variety of ways- oral consumption, smoking, snorting and injection. Of these injecting drug use is the most potent as the drug directly enters the drug stream. Injecting drug users (IDUs) shoot the drug sitting in a group and share needles and syringes. Needles and syringes are often re-used and not always sterilized. Along with the needles and syringes, blood also passes from one to another. One HIV positive person in the group infects the rest of the group.
Many IDUs are also sexually active and they pass on the infection which they got through their injecting drug use to their spouses as well as other commercial sex workers whom they might visit. Through these, sexual partners of the IDUs, the infection spreads to their other non-drug using sexual partners and clients as well.
Thus, injecting drug use spreads the HIV virus to persons who are not even remotely connected to abuse of drugs.
The National AIDS Control Organization (NACO) rates IDUs among the most important target groups vulnerable to HIV infection. For details of extent of HIV among different groups of people including IDUs, please see NACO website.