American children in the late twentieth century were still malnourished and physically unfit. But a group portrait of their general health contained one stunningly positive change: a greatly lessened threat from infectious and contagious illness. In 1900, sixteen out of every one hundred American children died from disease before reaching the age of five. Through the 1920s, more than fifteen thousand youngsters annually suffered agonized deaths from diphtheria alone. Their rasping gasps as they fought for breath were a familiar, and terrifying, sound, as was the high-pitched wail that gave pertussis its common name, whooping cough.

By the end of the century, vaccines defeated diphtheria, pertussis, and many other childhood enemies. In 1999, no American child died of diphtheria. All fifty states required that children be immunized against seven preventable diseases, and an estimated 97 percent of American schoolchildren received vaccinations against diphtheria, tetanus, pertussis, polio, measles, mumps, and rubella (German measles) by the time they began the first grade.

After 1985, new vaccines to protect against other childhood infectious diseases appeared. One prevented Haemophilus influenza type b (Hib) – which, despite its confusing name, was not a flu virus but a bacterium that could cause meningitus, epiglottitis, croup, pneumonia, and severe infections of the heart and soft tissues. Others protected against the Varicella virus (chicken pox) and Hepatitis strains A and B. In 1998, the Food and Drug Administration (FDA) approved a vaccine against yet another childhood illness, rotavirus, a diarrheal disease that annually sickened an estimated 3.5 million American babies.