Explain why it can be difficult to definitively diagnose a disease.

The disease can mutate to avoid the diagnosis and/or medication given to treat the disease.

Because heroin and LSD have a high potential for abuse and no current accepted medical uses, they are identified as A. schedule I drugs.
This means that these drugs are unsupervised, and can lead to severe problems if used constantly. Apart from heroin and LSD, peyote, methaqualone, and ecstasy fall under this category as well. The other types of drugs are not as sever as this one.