A needle-exchange program focused on a neighborhood with a high number of intravenous drug users is an example of prevention.

Secondary

Explanation:

There are several levels of prevention. You might confuse this with primary prevention. Primary prevention is preventing infection, so it has to be done before the patient caught the disease.

The needle-exchange program aims to reduce the spread of blood-borne disease(HIV, hepatitis, etc) from the multiple usages of the same needle. Since the disease already exists, this will be secondary prevention.