Why are heroin users at a higher risk for contracting hiv/aids and hepatitis b and c?

Contaminated needles. Years ago they found a case of hepb in a worker who got stuck by a needle cleaning out a crack house that had been abandoned for a year. All the mentioned illnesses are passed readily thru contaminated needles. The blood tainting the needle itself, hub or syringe can spread the disease to any subsequent user.
Intravenous use. The most common way heroin is abused is by injecting it into a vein. The user takes unsterile drug, puts it into unsterile water (or even spit or urine) and then uses a syringe and needle (as well as the "cooker") that has often been used by others. If the germs for HIV or hepatatits b/c are present anywhere along the line, infection is likely to happen. Same is true of cocaine, amphetamine etc.

Related Questions

Could the HBV vaccine help protect you against hiv/aids?

No. This agent is for the prevention of a separate virus. It would provide no benefit except to prevent the target virus. Read more...