The greatest vaccine success story of the modern age involves smallpox. Smallpox was a highly contagious viral illness that killed over 25 percent of those who were infected and left survivors maimed and disfigured. Fortunately, smallpox only infected humans, spread relatively slowly, and could be easily prevented by vaccination. Thus, it was possible to prevent the spread of the disease by immunizing enough people and quarantining affected individuals while contagious.
Starting in the 1960s, the World Health Organization (WHO) made a concerted effort to isolate epidemics of smallpox and vaccinate entire populations in the areas surrounding the epidemic. The number of cases of smallpox worldwide declined from estimates of over two million in the 1960s to zero by the late 1970s. In 1980, the WHO officially certified the world free of smallpox, and the vaccine is no longer given.
Another vaccine success story is polio. The number of cases of paralytic polio worldwide has decreased from three hundred thousand in the late 1980s to less than two thousand cases in the year 2006. This progress is mostly due to a global eradication program that began in 1988. Unfortunately, polio will be more difficult than smallpox to eradicate. The polio virus can cause not only the severe, paralytic form of the illness but also a milder, more common presentation. Most individuals with the milder form do not even know they have the disease and can thus continue to spread it to others through their bowel movements. In areas of poor sanitation, this is a common form of transmission.