How do needle exchange programs not effectively prevent disease?

NEP's allow people to bring in their dirty needles and exchange them one-for-one for clean, sterile ones at absolutely no cost. I cannot possibly see how this doesn't effectively prevent disease and I would love to know how that is possible.How do needle exchange programs not effectively prevent disease?Don't know what you're asking. this sort of program does prevent disease quite well - but America is such a christians-self righteous nation I think a lot of people want to see drug addicts dying of diseases if possible.How do needle exchange programs not effectively prevent disease?A needle exchange would fail only not taken advantage of. I'm certain that there are still people out there too suspicious, too messed up or too lazy to participate in an NEP.