"Posters have been a powerful force in shaping public opinion because propagandists have long known that visual impressions are extremely strong. People may forget a newspaper article but most remember a picture. A pamphlet or a newspaper can be thrown away, unread; the radio or television turned off; films or political meetings not attended. But everyone at some time or other notices messages when walking or driving, or sees posters on bulletin boards in offices, hospitals, clinics or pharmacies. The main objective of posters, as with other communications media is to influence attitudes, to sell a product or service or to change behavior patterns. Public health posters are clearly in the third category, their purpose being to alter the consciousness of the public to bring about an improvement in health practices."

-- William H. Helfand, To Your Health: An Exhibition of Posters for Contemporary Public Health Issues, National Library of Medicine