It has quelled outbreaks of Ebola, allowed smallpox to be corralled before being vanquished by a vaccine, and helped turn HIV into a survivable illness. And whenever a new infectious disease emerges, contact tracing is public health’s most powerful weapon for tracking transmission and figuring out how best to protect the population.

But now, as coronavirus cases are surging in hot spots across the country, the proven strategy’s efficacy is in doubt: Contact tracing failed to stanch the first wave of coronavirus infections, and today’s far more extensive undertaking will require 100,000 or more trained tracers to delve into strangers’ personal lives and persuade even some without symptoms to stay home. Health departments in many of the worst-affected communities are way behind in hiring and training those people. The effort may also be hobbled by the long-standing distrust among minorities of public health officials, as well as worries about promising new technologies that pit privacy against the public good.

“We don’t have a great track record in the United States of trust in the public health system,” said David C. Harvey, executive director of the National Coalition of STD Directors. Ever since the 40-year Tuskegee experiment, which withheld treatment for syphilis from poor black men, officials have had to make special efforts, he said, to reach those now “disproportionately impacted by covid who are African Americans and Latinos.”

Still, as states relax restrictions, public health experts believe wide-scale contact tracing is the price that must be paid to reopen safely without reverting to the blanket lockdown that put nearly 40 million Americans out of work. Time is of the essence, they say, taking advantage of the drop in cases resulting from the shutdowns.

“Contact tracing is finding the next generation before they happen, getting ahead of that transmission cycle to stop it,” said Emily Gurley, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health and the instructor of the school’s new six-hour online contact-tracing course. Gurley doesn’t believe the strategy will stop transmission, but that in concert with testing and other measures, it can prevent the disease from spreading exponentially.

In this Tuesday, May 19, 2020, photo, health investigator Mackenzie Bray sits in her office at the Salt Lake County Health Department in Salt Lake City. Bray normally works to track contacts for people with sexually transmitted diseases, but she was re-assigned during the coronavirus pandemic. She is now one of 130 people at this county health department assigned to track down COVID-19 cases in Utah’s urban center around Salt Lake City. AP Photo/Rick Bowmer

Right now, though, the virus is showing signs of taking the lead again. As states have lifted restrictions on retail and large gatherings, more than a dozen are facing new heights in cases or hospitalizations, according to Washington Post data.

In Arizona, for example, the governor reopened before local health departments had hired and trained its new army of contact tracers, said Will Humble, former director of the state’s health department.

“We flattened the curve. Then, by the time we ended, the contact tracers weren’t up and running yet,” said Humble, who described case investigation and contact tracing as key elements of a multipronged response, including mask-wearing and social distancing. The health departments in the state’s hard-hit urban counties have been repurposing staff, in addition to making new hires, he said, using federal dollars and support from an Arizona-based nonprofit group, the Crisis Response Network.

Incentives could have been built in, tying each region’s reopening to its hiring of adequate contact tracers, Humble said.

“We didn’t do that here,” he said. “Now we have to ramp up a contact-tracing workforce that isn’t going to get to everything probably.”

Texas, also seeing a dramatic surge, has relaxed restrictions after hiring about 3,000 of the 4,000 contact tracers Gov. Greg Abbott, a Republican, said in April he planned to have in place as part of his reopening strategy.

“Both we and the local health departments continue to add staff,” said Chris Van Deusen, spokesman for the Texas Department of State Health Services. “We can scale up further if that becomes necessary.”

Michael Sweat, director of the Center for Global Health at the Medical University of South Carolina, said the state health department, which has suffered from long-term underfunding, was trying hard to ramp up contact tracing as parts of the state suffer “worrisome micro-epidemics.”

“There’s a lot of effort going into training and deploying people, and working on technology to help. But they are still getting their footing,” Sweat said, as the infection growth rate in Charleston suddenly doubled.

In April, the Centers for Disease Control and Prevention awarded $631 million from the Cares Act to state and local health departments for surveillance, including contact tracing, even as a report from the Association of State and Territorial Health Officials and the Johns Hopkins Center for Health Security estimated 100,000 new hires will be needed to trace all contacts, safely isolate the sick and quarantine those exposed, at a cost of $3.6 billion.

Across the country, the efforts to ramp up are vast and varied.

The University of California at San Francisco has been tapped by the state to create a Pandemic Workforce Training Academy that will train as many as 3,000 people for the state’s 58 county health departments, many of them focusing on low-income communities where requests to quarantine can be financially devastating.

In Fairfax County, Virginia, the county health department has subcontracted to a private company, GattiHR, to create a 400-strong, largely remote contact-tracing team, looking for people with “empathy, attention to detail, resilience [and] investigative skills,” and finding successful applicants among those furloughed from the hospitality industry.

In Rhode Island, Gov. Gina Raimondo, a Democrat, unveiled a free voluntary app that health officials hope will prove more reliable than people’s memories in re-creating their recent contacts – one of numerous cellphone tracking innovations, including the Apple-Google exposure notification system, that have prompted privacy concerns from civil libertarians.

And in Florida, the Coalition of Immokalee Workers has been working urgently with Doctors Without Borders to win the confidence of migrant workers, where 37 percent of those tested at pop-up clinics were positive. Their goal is to slow the virus’s spread before farmworkers leave for summer jobs in Georgia, South Carolina and beyond.

“We have a window of opportunity,” said Gerardo Reyes Chavez, a former farmworker, who said that as people have become sick, they have worried they might lose their jobs. “They are having to weigh what is scarier for them – to know they have the disease or not.”

But several people like Chavez who work with immigrant groups said people have grown more willing to respond to contact tracers as the virus has sickened more people, giving them concerns about infecting their own family members.

In San Francisco, librarian Ramses Escobedo, who became a contact tracer after two weeks of training, said the health department gave out a 60-page instruction document. “It has information from the scripts you’re supposed to follow, the questions you’re supposed to ask.” (Escobedo, who speaks Spanish, noticed some errors in the Spanish translations and said he had them fixed.)

Of the 30 people Escobedo spoke to in his first three weeks as a contact tracer, only one refused to answer his questions.

Susie Welty, a Spanish-speaking contact tracer who joined the UCSF effort after her own overseas research on HIV was suspended by the pandemic, also said people have largely been responsive. Getting them to agree to voluntarily self-isolate is far easier when resources are available to provide food and other out-of-pocket payments during the 14-day period.

Welty described a conversation with a pastor whose wife had tested positive. When the pastor explained he did not want his congregation to know and so did not want them bringing food during their quarantine, Welty was able to refer them to SNAP, the food supplement program.

“San Francisco has resources,” Welty said. “That is not the case in many jurisdictions,” she added, saying it is particularly hard for undocumented workers to comply ifthey are unable to feed their families.

“They’re scared,” said Venus Ginés, founder of the Latino community health organization Dia de la Mujer Latina, which operates in Houston and other cities.

After the Houston health department asked Ginés to help fill contact-tracing positions, her organization supplied 200 résumés of Spanish-speaking applicants within 24 hours, and Ginés said the health department told her that hires will be made from that pool.

Kirstin Short, Houston Health Department bureau chief of epidemiology, said her agency relies on Dia de la Mujer Latina and other organizations to “speak as that trusted authority within that community to vouch for us as a government entity.”

But the possibility of data falling into hostile hands worries representatives of immigrant groups.

“There’s always that fear if I say something, and this person is undocumented and the government finds out about it, then that person could be deported,” Ginés said. “We don’t know if this information is going to get hacked or how it is going to be utilized.”

Using cellphone location data or Bluetooth to determine proximity, as has been done successfully in Singapore and South Korea, increases those concerns.

The app rolled out in Rhode Island is voluntary – an effort to walk a line between digital data collection and protecting civil liberties.

“Privacy and data protection are paramount,” Raimondo said in an interview. “Which means I need to give you confidence that if you opt in, your data is safe.”

But privacy advocate Mitchell Baker, CEO of Mozilla, which owns the Firefox Web browser, said it’s easy for data to be misused.

“How do citizens know what is actually happening? What data are you collecting, where is it going, how is it used, and when and how is it destroyed?” she asked.

Not everyone owns a smartphone. And many experts believe conversations with a contact tracer are preferable for other reasons, too. When a disease so disproportionately affects marginalized populations, it’s important to build trust, said Joia Mukherjee, chief medical officer of the international medical nonprofit Partners in Health, which works with the Massachusetts contact-tracing program, as well as numerous vulnerable communities across the country.

“This is a terrifying time. You need some level of human contact,” Mukherjee said. “If I were exposed, I would want to know to protect my mother’s life.”

Even when people do comply – in person, on the phone or online – the novel coronavirus is proving to be an exceptionally wily foe.

Unlike a blood-borne or sexually transmitted disease, or one such as smallpox or measles that scars its victims, the coronavirus moves invisibly on people’s breath, meaning they may have no idea they’ve recently spent time with somebody who is infected.

Because people can become contagious in just a few days – as opposed to two weeks for syphilis, for example – contact tracers have limited time to reach them before the virus moves on, leading some epidemiologists to believe digital technologies are key to stopping it.

And the microscopic bug moves so stealthily – before symptoms show up, or without them ever appearing – that it confounded the earliest attempts to corner it, according to a recent CDC report. This past week, after a World Health Organization official cast doubt on whether the virus could spread asymptomatically and then revised her position, doctors called for more clarity on anissue withsuch profound implications for how they practice medicine.

The best way to establish the truth, infectious disease specialists say, is to use contact tracing to build a fuller picture of the virus’s habits and preferences, including information about people who for some reason escape infection.

“It’s not a silver bullet: It won’t reach everyone; not everybody will comply.” Welty said. “But it’s the best we have now, the best we will have until we have a vaccine.”