How likely are you to answer an unknown number popping up on your caller ID? And if you were to pick up, how likely is it that you would disclose personal information about yourself and your family, friends and colleagues to the stranger on the other end of the line?Ask any census worker, public opinion pollster or cold caller, and they will tell you the answer to both questions is “not likely.” Indeed, the pickup rate for unfamiliar or unidentified numbers in those circumstances is as low as 6 percent. And the people who do answer are often wary and uncooperative, if not abusive.That resistance is the crux of the problem with contact tracing, which health experts say is essential to containing the coronavirus.The idea is to reach out to everyone who has tested positive for the virus, find out whom they might have inadvertently exposed and encourage them all to avoid infecting others. But an army of 180,000 contact tracers provisioned with telephone headsets and scripts does not guarantee that anyone will want to talk to them, much less follow their advice.A storied group of venereal disease investigators from the last century provides some clues on what it might take for contract tracing to be effective and whether the current patchwork of city and state programs is missing the mark.“The best contact tracers that we have ever had came out of the V.D. investigation program started after World War II,” said Dr. William Foege, a former director of the Centers for Disease Control and Prevention who is credited with devising the global strategy that led to the eradication of smallpox in the 1970s.This was an elite cadre of carefully vetted and trained individuals, who, starting in 1948, were tasked by the Public Health Service to contact trace cases of venereal disease, particularly syphilis. Syphilis was a scourge back then; during wartime, approximately 25,000 positive tests were reported annually in New York City alone.The shame associated with syphilis made it especially difficult to get infected people to talk. And tracking down contacts was a challenge when often all you had to go on was, “the girl who frequents the red-door bar downtown” or “a guy who said he was a brush salesman from Topeka.” Nevertheless, public health advisers, or P.H.A.s, as the investigators were called, were astoundingly effective. By the mid-1950s, syphilis rates were the lowest they had ever been. (Also of note, it was a relentless P.H.A. who blew the whistle on the infamous Tuskegee experiment.)“They had to be psychiatrists, detectives and problem solvers all at once,” Dr. Foege said. He worked closely with many public health advisers who ended up at the C.D.C. spearheading efforts to contain other infectious diseases, such as smallpox, measles, H.I.V./AIDS, cholera and Ebola.The boss of the operation was the poker-playing biostatistician Lida Usilton. Under her direction, all P.H.A.s went through the same selection process. They had to have a college degree, liberal arts preferred, and a variety of work experiences and backgrounds was a plus. One put himself through school working as a dishwasher and a lumberjack. Another was a former farmhand and football player. Some were prisoners of war.The thinking was that contact tracers had to be able to talk as easily with a Wall Street banker as with a migrant worker. They were interviewed extensively to see whether they had the kind of emotional intelligence that made people want to talk to them and whether they could easily manage conversational curveballs, like when a female contact revealed she was biologically a man.“They didn’t have to be rocket scientists but they had to be able to connect with other people — to talk in a way that indicated to us they were very approachable and reasonable human beings who could be sensitive and persuasive,” said Frederick Stuart Kingma, a 92-year-old retired P.H.A. who trained and recruited other tracers in the 1950s.Today the process is different. Coronavirus tracers are hired according to various state or local health authorities’ requirements, for jobs paying from $15 to $30 an hour. In New York City, where 10,000 people have applied for 2,500 coronavirus contact tracing jobs, officials said preference was being given to clients of local food-assistance and social-support organizations. In San Francisco, they are hiring primarily furloughed city employees, such as librarians, city attorneys and tax assessors. And in Massachusetts, where 45,000 people applied for 1,700 positions, résumés were sorted first by algorithm and final decisions were based in large part on videos that applicants submitted explaining why they wanted the job.“We’re still learning who the best people are to do this,” said Dr. John Welch, who is overseeing recruitment and training of contact tracers for Massachusetts in collaboration with the nonprofit Partners in Health. “A furloughed doctor or nurse may not have that empathetic ear that’s critical.”Training is similarly all over the map, with some health agencies requiring online courses lasting a few hours and others demanding eight to 20 hours of in-person training, or a combination of both. Much of the training focuses on education about the coronavirus, confidentiality rules and data entry protocols. Relatively little instruction is given on how to build rapport with and earn the trust of people, particularly when they might be reluctant to reveal their contacts, like, say, a married lover or an undocumented employee.For example, just one hour of the six-hour online training program developed by Johns Hopkins Bloomberg School of Public Health, which New York City and the state of New York are using, is devoted to interviewing techniques. It recommends memorizing catchphrases to use during calls such as “I hear you” and “This is a difficult time.” That is, the kinds of canned responses that might send someone into a blind fury when calling tech support.Dr. Emily Gurley, an infectious-disease epidemiologist who developed the Bloomberg training, said that if people realize that the person on the other end of the line is trying to help them, “then I think your interviewing skills don’t need to be that great to really get and give the information that you need to.”P.H.A.s, by contrast, were taught that everything depended on their interviewing skills. They were sent away to a kind of interpersonal-skills boot camp where they spent an intense two weeks learning interviewing techniques, shadowing other contact tracers and interviewing people themselves under close supervision. Their success in the field was then closely monitored in terms of numbers of cases reached and contacts generated, as well as how many contacts got tested and treated.An informal survey of coronavirus contact tracing programs across the country revealed that none had settled on definite metrics of success. “We are looking for a set of various measures,” said Dr. Umair Shah, the executive director of Harris County Public Health in Texas, which serves Houston and the surrounding area. “But ultimately the juice is worth the squeeze — if you’re only reaching 5 percent of the people, at some point you have to ask why and decide maybe this is not the tool we should use.”This brings us back to getting people to pick up the phone, on which so much depends. Some health authorities are sending text messages first, encouraging people to answer, while others are working on social media campaigns and radio jingles to raise awareness and improve participation. It recalls P.H.A.s in the 1950s driving around the countryside broadcasting songs about syphilis over loudspeakers. They passed out comic books about venereal disease with titles like “Little Willie” and “That Ignorant Cowboy” and gave out free samples of Dentyne chewing gum to anyone who got tested.“It was a long time ago,” said Mr. Kingma, who, some 70 years after working as a P.H.A., can still make you want to tell him everything moments after getting on the phone with him. “But maybe some of the things we learned back then can be of use.”Kate Murphy is a journalist in Houston who contributes frequently to The New York Times and the author of “You’re Not Listening: What You’re Missing and Why It Matters.”The Times is committed to publishing a diversity of letters to the editor. We’d like to hear what you think about this or any of our articles. Here are some tips. 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