In layman's terms, "disease involving the white matter of the brain turning into spongy mush." The most affected area is the cerebellum, which controls coordination. As of now, the most popular cause of the disease is the drug practice known as "chasing the dragon," where heroin is heated on foil and inhaled.
It has an estimated mortality rate of 23%, and can be reversible if caught in time. No cases have ever turned up involving other methods of use, such as intravenous injection or intranasal inhalation (aka "snorting"). No one really knows what causes the disease, but there is a highly established link with smoking heroin. As far as I know, no one has done a study to determine if it's the foil that causes the problem, as the current theory is that heating the heroin forms a host of by-products of combustion that just happen to really damage the inner brain. There may be one or two principal chemicals that cause the problem, possibly through reacting with foil. Or, it could simply be that artificially synthesized (either with natural precursors or from the ground up) heroin was never meant to be smoked, but who's doing to do the research to comfort those who would like to partake in the practice?
Unfortunately this means that there really is no safe way for a heroin user to dose, unless the drug is provided in 100% pure form by the government or a reliable health care provider. Nasal inhalation can be dangerous if the purity of the dosage is not well-known or properly controlled. Intravenous injection can introduce a host of dangerous infections, as well as impurities from the adulterants added to the cut the heroin and increase its street value.
Just another reason to avoid heroin, not that I'm knocking any one who really likes it. Heroin is on par with cigarettes in terms of addiction, but there are people out there who can manage to do it on a regular or sporadic basis without every being addicted to a point where it makes life difficult for them. If you really enjoy chasing the dragon, perhaps you should try pure opium (and not on foil), as there haven't been any cases of that causing the aforementioned disease. The only reason we don't hear about tobacco addiction destroying lives financially is because it costs $3 for 20 fixes!