The polio campaign is a prime example, as evidenced by the backlash after the Bin Laden killing. As recently as the 1980s, the disease paralyzed an estimated 350,000 people a year in 125 countries. Today it is corralled in just three nations: Afghanistan, Nigeria and Pakistan. Last year, W.H.O. reports, the worldwide total was a mere 32 cases. Still, getting to zero has proved elusive.

Much the same may be said about other infectious diseases that are candidates for eradication, notably measles and rubella. People keep getting in the way. “Lack of sufficient political will is probably the greatest impediment to global measles eradication,” the National Center for Biotechnology Information said in 2002.

In this country, resistance to vaccinating children has emerged even among some who are well educated and affluent. But inaction can have consequences, like an outbreak of measles that struck Disneyland in Southern California two years ago and spread across the state and beyond.

Some Somali immigrants in Minnesota, having bought into the widely discredited theory that the vaccine causes autism, refused to have their children inoculated against measles, mumps and rubella. The result has been Minnesota’s most severe flare-up of measles in several decades. Likewise, vaccine skepticism has taken a toll among Europeans, notably in Romania and Italy, both of which are experiencing an explosion of measles cases.

An outbreak of Ebola that killed more than 11,000 people in West Africa in 2014 illustrated the good that can be done if political will does exist. While Sierra Leone, Liberia and Guinea were hit hard, Nigeria emerged relatively unscathed. That’s because the authorities there adopted an aggressive approach in cooperation with agencies like W.H.O., Doctors Without Borders, Unicef and the United States Centers for Disease Control and Prevention.

Health professionals intensively tracked anyone who might have had contact with infected patients, starting with the first known case in that country, a Liberian-American who had flown into Lagos. Nearly 900 such people were identified, and each was visited an average of 20 times. Suspected cases were thus isolated. As a result, the Nigerian death toll from Ebola was held to single digits.

New threats keep coming. The C.D.C. has monitored more than 300 outbreaks of various diseases in 160 countries. Infectious illnesses are believed to cause 15 percent of all deaths worldwide.

In President Trump, the United States may well have its most prominent germaphobe, but his commitment to public health seems thin. His proposed budget for the current fiscal year included sharp reductions in domestic spending on medical research, and in aid for public health projects overseas. Congress was unimpressed. It not only rejected most of his desired cuts, it also set aside an extra $2 billion for the National Institutes of Health. There is no guarantee, though, that the president won’t try slicing the research budget yet again in 2018.

For his part, Dr. Brilliant emphasizes that the key to beating back an infectious disease is “early detection, early response.” He utters the phrase as if it is a mantra. But in a strife-prone world, translating those words into action is as big a challenge as ever.

CLYDE HABERMAN, a regular contributor to Retro Report, has been a reporter, columnist and editorial writer for The New York Times, where he spent nearly 13 years based in Tokyo, Rome and Jerusalem. Subscribe to our newsletter here and follow us on Twitter @RetroReport.

This article first appeared in The New York Times.