The year is 1952, and hysteria is sweeping across the United States. What is everyone afraid of? Polio.

That year, the disease, which targets children, sickened nearly 58,000 Americans, killed more than 3,000 and left more than 20,000 with mild to disabling paralysis. That summer, at the height of the epidemic, public pools were closed, movie theaters were shuttered and infected children were placed under quarantine — involuntarily, if necessary.

A decade later, the average number of polio cases in this country dropped to about 900 per year. In another decade, polio was eliminated from the United States. What caused polio to vanish? Vaccination. Globally, cases of polio have been reduced by 99 percent, thanks to the polio vaccine.

Immunization is a medical miracle that is key to the steady progress in fighting the world’s communicable diseases. Each year immunizations prevent more than 2.5 million deaths just through vaccination against four diseases: diphtheria, tetanus, pertussis (DTP) and measles.

But complacency is a danger, as we are seeing here in the United States with the spike in measles, largely due to the decision of parents not to vaccinate their children. The progress we’ve made can always be lost, and the World Health Organization notes that there are still 20 million unvaccinated and under-vaccinated children in the world today.

The benefits of immunization are multifaceted. On the individual level, they improve general health by preventing disease. But they also contribute to children’s overall development by avoiding illnesses that would impair cognitive ability and physical strength and would keep them from attending school for long stretches. On a community level, preventing disease avoids lost productivity, both for the patient and the caregiver.

In addition, immunization provides an opportunity for children and their families to avail themselves of primary health services several times during a child’s first years of life. Immunization outreach efforts provide health professionals with opportunities to reach communities with other services, including malaria prevention, prenatal and neonatal health care, and sexual and reproductive health education, among others.

Immunization programs also bring benefits to a community’s health system. Health workers receive training that stays with the community. And because some vaccines must be kept below a certain temperature to stay effective, developing the infrastructure and logistics to ensure cold storage improves the health system in ways that have wider benefits.

Administering a national immunization program in the developing world provides some significant challenges, which makes the progress achieved all the more astounding. Many of the poorest and most vulnerable people in developing countries live in rural areas, requiring long journeys from urban areas over poor or nonexistent roads. In many of these isolated areas, there are few or no hospitals, let alone health clinics and health care workers to administer vaccines. Resources are needed to build or strengthen the health care system to reach these communities.

Armed conflict is another barrier to immunization. Wars and civil unrest present dangerous conditions for health care workers and the people they are trying to serve. And conflict is driving people to leave their homes and communities in one of the highest levels of forced migration ever recorded, making consistent health care delivery extremely difficult.

Despite these challenges, the goal of drastically reducing or eliminating some of the world’s most deadly diseases is a possibility. According to recent WHO data, about 85 percent of infants worldwide had received the required three doses — known as the DTP3 — of the vaccine against diphtheria, tetanus and pertussis, and 123 countries had reached at least 90 percent coverage with the DTP3 vaccine.

Progress has also been encouraging vaccination against measles, a potentially fatal disease whose crippling effects have faded from memory in the U.S. Worldwide, 85 percent of children had received one dose of measles vaccine by their second birthday, and 67 percent of children received two doses.

But now is not the time to revel in what we have achieved. We must redouble our efforts, now that the goal of eradication of some of the most lethal and crippling diseases — unthinkable in the recent past — is in sight.

Allyson Bear (allysonbear@imaworldhealth.org) is the vice president for health programs at IMA World Health and Lutheran World Relief and William Moss (wmoss1@jhu.edu) is executive director of the International Vaccine Access Center at Johns Hopkins Bloomberg School of Public Health.