BaltimoreMarylandLongitudinal Studies: Studies in which variables relating to an individual or group of individuals are assessed over a period of time.District of Columbia: A federal area located between Maryland and Virginia on the Potomac river; it is coextensive with Washington, D.C., which is the capital of the United States.Urban Population: The inhabitants of a city or town, including metropolitan areas and suburban areas.African Americans: Persons living in the United States having origins in any of the black groups of Africa.Volunteers: Persons who donate their services.Substance Abuse, Intravenous: Abuse, overuse, or misuse of a substance by its injection into a vein.Fibrinogens, Abnormal: Fibrinogens which have a functional defect as the result of one or more amino acid substitutions in the amino acid sequence of normal fibrinogen. Abnormalities of the fibrinogen molecule may impair any of the major steps involved in the conversion of fibrinogen into stabilized fibrin, such as cleavage of the fibrinopeptides by thrombin, polymerization and cross-linking of fibrin. The resulting dysfibrinogenemias can be clinically silent or can be associated with bleeding, thrombosis or defective wound healing.Aging: The gradual irreversible changes in structure and function of an organism that occur as a result of the passage of time.Small Business: For-profit enterprise with relatively few to moderate number of employees and low to moderate volume of sales.Dancing: Rhythmic and patterned body movements which are usually performed to music.Residence Characteristics: Elements of residence that characterize a population. They are applicable in determining need for and utilization of health services.Needle-Exchange Programs: Organized services for exchange of sterile needles and syringes used for injections as a potential means of reducing the transmission of infectious diseases.Transfer Agreement: A written agreement for the transfer of patients and their medical records from one health care institution to another.Drug Users: People who take drugs for a non-therapeutic or non-medical effect. The drugs may be legal or illegal, but their use often results in adverse medical, legal, or social consequences for the users.Catchment Area (Health): A geographic area defined and served by a health program or institution.