Heroin is one of the most addictive drugs on earth. Stronger, purer and easier to get than ever before, heroin is attracting a whole new breed of addict. Dangerously pure, the newest forms of heroin can be snorted or smoked, rather than injected, attracting millions of new mainstream users. National Geographic investigates how the war in Afghanistan has inadvertently unleashed a deluge of heroin, enticing users from Kabul to Europe to America's heartland.