Throughout history, devastating epidemics of infectious diseases have wiped out large percentages of the human population. In the mid-fourteenth century, the Black Death, a plague epidemic, killed roughly one-third of Europe's population. More recently, in 1918, an outbreak of the flu killed an estimated 20 million people, more people than died in all of World War I. In our own times, the acquired immune deficiency syndrome (AIDS) pandemic has brought untold personal suffering and social losses. The Centers for Disease Control (CDC) estimates that, from 1981 to 2001, approximately 21 million people died from AIDS worldwide. Millions of people all over the world are currently infected with the human immunodeficiency syndrome (HIV) virus, about 95% of them in developing countries.

Although medical advances have reduced the consequences of some infectious diseases, preventing infections in the first place is preferable to treating them. The development of vaccines gives us not only a means of protecting ourselves as individuals, but also, and perhaps more importantly from a public health view, a means of preventing sudden and widespread outbreaks.

Once a vaccine is developed, how should it be used? Should everyone in a society be required to be immunized for certain illnesses, regardless of their personal desires? Is the cost of an immunization program worthwhile if a vaccine is expensive or difficult to administer? If only those facing the highest risk of a disease are immunized, will that be sufficient to prevent epidemics?