The second most frequent mode of HIV transmission is via blood and blood products.[4] Blood-borne transmission can be through needle-sharing during intravenous drug use, needle stick injury, transfusion of contaminated blood or blood product, or medical injections with unsterilised equipment. The risk from sharing a needle during drug injection is between 0.63 and 2.4% per act, with an average of 0.8%.[51] The risk of acquiring HIV from a needle stick from an HIV-infected person is estimated as 0.3% (about 1 in 333) per act and the risk following mucous membrane exposure to infected blood as 0.09% (about 1 in 1000) per act.[32] In the United States intravenous drug users made up 12% of all new cases of HIV in 2009,[38] and in some areas more than 80% of people who inject drugs are HIV positive.[4]
HIV is transmitted in about 93% of blood transfusions using infected blood.[51] In developed countries the risk of acquiring HIV from a blood transfusion is extremely low (less than one in half a million) where improved donor selection and HIV screening is performed;[4] for example, in the UK the risk is reported at one in five million.[52] In low income countries, only half of transfusions may be appropriately screened (as of 2008),[53] and it is estimated that up to 15% of HIV infections in these areas come from transfusion of infected blood and blood products, representing between 5% and 10% of global infections.[4][54]
Unsafe medical injections play a significant role in HIV spread in sub-Saharan Africa. In 2007, between 12 and 17% of infections in this region were attributed to medical syringe use.[55] The World Health Organization estimates the risk of transmission as a result of a medical injection in Africa at 1.2%.[55] Significant risks are also associated with invasive procedures, assisted delivery, and dental care in this area of the world.[55]
People giving or receiving tattoos, piercings, and scarification are theoretically at risk of infection but no confirmed cases have been documented.[56] It is not possible for mosquitoes or other insects to transmit HIV.