One child with measles will infect 9 out of 10 unvaccinated people around them—a staggering reminder of why a disease nearly wiped from the American landscape could return in an instant. Yet across the United States, vaccines have quietly performed one of medicine's greatest feats: transforming 14 once-devastating childhood illnesses into medical curiosities, rare enough that many parents today have never seen a child suffer from them.
This success story matters more now than ever. As vaccination rates decline in certain communities, we're witnessing outbreaks of diseases that seemed vanquished—measles, mumps, and pertussis have all surged in recent years, a sobering signal that immunity depends on sustained effort. When enough people get vaccinated, these diseases lose the ability to spread, protecting everyone including those too young or medically fragile to be vaccinated themselves.
The mathematics of measles illustrate just how vulnerable we are without vaccines. Two doses of the MMR vaccine are 97% effective at stopping the disease, yet a single unvaccinated child in a classroom can trigger an outbreak. Babies under one year old—too young for their first shot—face particular danger; measles doesn't just make them acutely ill, it can obliterate their immune system's "memory," leaving them defenseless against other infections. The virus remains common in other parts of the world, meaning unvaccinated travelers can inadvertently import it back to the United States.
Pertussis, or whooping cough, earned its nickname honestly. The "100-day cough" describes an illness so relentless that infected children can cough for months, sometimes producing a distinctive high-pitched gasp for air between fits. For infants, it's dangerous. About one-third of babies under 6 months old with whooping cough require hospitalization; their small airways and developing lungs simply cannot handle the repeated coughing spells. Newborns, too young to have started their five-dose DTaP vaccine series, are most vulnerable—which is why pregnant women getting the Tdap vaccine becomes an act of protection for the unborn.
Mumps might seem like a quaint, historical disease, but its complications remind us why prevention beats cure. Swollen salivary glands give the illness its distinctive puffy-cheeked appearance, but the virus can trigger meningitis, permanent deafness, and infertility. The MMR vaccine proved so effective that by 1999—just a decade after mumps vaccine became part of the combined shot in 1989—only one in a million American children caught the disease. Yet recent years have brought troubling outbreaks traced to exposures in classrooms and on sports teams.
Chickenpox presents a similar pattern. Before the varicella vaccine arrived in 1995, the virus was nearly universal among American children by age 9, causing a week or more of missed school and misery. Worse, infections frequently spread to children's lungs or brain, causing serious secondary illnesses. Since vaccination began, chickenpox infections have plummeted 97%. What once meant hospitalization—sometimes intensive care—for thousands of children annually has become genuinely rare.
These aren't abstract statistics. They represent children who play uninterrupted, who learn without fever, who breathe easily. Behind each vaccine success lies decades of medical research and millions of doses administered safely. The 14 diseases on this list share one truth: they haven't disappeared because they couldn't be stopped, but because enough people chose to stop them. As vaccination hesitancy emerges, that choice becomes increasingly urgent.