Either operate on language causation, or use a set of abstract concepts ("risk", "death", "disgrace", the name of the symptoms and much of what the child can not learn by associating with their sensory experience, ie, as he knows and wants to do). Talking with your child in the language of logic, the adults do not think about the simple fact that children are not available in this language fully, they are just starting to learn. Imagine that you need talk to a foreigner whose language you do not own. Perhaps you are using the following strategy: start talking with him in his language, using the help of intermediaries (dictionary, translator), but still on his tongue; using non-verbal means of communication (body language, emotions). To do this, you must refuse to use in explaining the abstractions of terms. As a practical exercise, try to explain to a child (you can imaginary) that he found in the courtyard of the used syringe is a real danger not only his health but life.
At the same time to explain better to forget abstractions such as "death", "contagious" hiv infection and aids. Just do not forget the main rule – to explain the inexplicable is only possible with the help of a friendly, that was already in the sensual experience of the child. So, explain to your child in a language he understands, and the level of development. There are lots of ways to do this: Very young children understands body language, facial expressions, intonation, and emotional manifestations.