The sharing of infected needles is the number one cause of new HIV cases in the United States. Drug users share dirty needles primarily because there simply are not enough clean ones to go around.
Needle exchanges make clean needles readily accessible, removing the reason to share.
By decreasing the sharing of needles, needle exchanges shrink the channels of transmission for HIV, helping to prevent its spread. Preventing the spread of HIV through infected needle-sharing doesn't just protect those who use or are addicted to intravenous drugs. The children and sexual partners of intravenous drug users are also directly threatened by the sharing of dirty needles. Thus needle exchanges are not simply a service rendered to intravenous drug users, but truly a public health measure intended to work as part of the nation's comprehensive HIV prevention plan. Though they do not receive the attention that HIV and AIDS do, they too are widespread in the injection drug using community.