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.

See, for example, Arabian Nights: "Night was beautiful, the sky was starry, and Sheherazade took out a condom."

Related posts from the past:

American Social Hygiene Posters, ca. 1910-1970

Public Health Posters at the National Library of Medicine

And these:
American Propaganda Posters from World War II (now here)

AIDS Posters - UCLA Digital Library