You're more likely to pick up blood borne diseases from sharing equipment (spoons, tourniquets, syringes - even tubes/rolls of paper for snorting) than you are from drugs themselves.
However, if you're dealing with drugs that have come in contact with someone's used IV equipment (such as a solution of drugs dissolved in water that may have been drawn by a used needle (or in a spoon previously used by another user) then there is a definite possibility of infection.
Drugs themselves - dry powders, especially - are unlikely to be able to carry HIV or Hep C.