For 20th-century Americans, an outbreak of the small, itchy blisters of chickenpox was an expected milestone on the path of childhood -- much like the first knee-skinning after learning to ride a bike. Times have changed: Although the chickenpox virus once infected 4 million Americans each year, today it sickens only 1 in 5,000 annually. The Centers for Disease Control and Prevention announced in August that the incidence of chickenpox dropped 97 percent between 1995 and 2010, due to the introduction of the chickenpox vaccine.
It's a similar story for a more dangerous disease -- measles. It decreased sharply after a measles vaccine came into widespread use several decades ago. Health officials declared measles eliminated from the United States in 2000, but the virus finds opportunities to sneak back and cause outbreaks because of gaps in immunization coverage....
