This coronavirus course will train an army of contact tracers. You can take it, too.

The nation needs a sprawling network of contact tracers to track and halt the spread of COVID-19, the disease caused by the coronavirus. Public health experts estimate that an effective tracking system will require at least 15 tracers per 100,000 Americans — and, in the hardest-hit regions, a workforce twice that size.

About $3.7 billion will be needed to cover the work of 100,000 tracers, the National Association of County and City Health Officials calculated. Creating this army of contact tracers, bigger than any assembled in U.S. history, will require swift, efficient training.

To that end, on Monday, the Johns Hopkins Bloomberg School of Public Health unveiled a course on the online platform Coursera to teach Americans the fundamentals of contact tracing.

"The motivation for doing the course is to be able to provide a good, solid, basic training for the workforce that needs to scale up very rapidly," said Joshua M. Sharfstein, a physician at the Johns Hopkins public health school and director of the Bloomberg American Health Initiative. Privacy, medical ethics, virology and interview techniques are included in the six-hour package. The course is available free of charge to the public.

Contact tracers notify possibly infected individuals and map their exposures to other people. They are also a link between the public health system and communities. "Contact tracers are in part detective, part therapist and part social worker," said Johns Hopkins epidemiologist Emily S. Gurley, the course's lead instructor.

The chain of transmission snaps if contagious people are isolated before they infect others. Contact tracing, an old tool in the public health arsenal, helped eradicate smallpox in the United States in the 1950s. Contact tracers more recently have been employed to follow sexually transmitted microbes, Ebola, leprosy and other diseases.

The coronavirus is difficult to track because it is speedy. “The incubation period is fast. This disease moves really fast, faster than Ebola or syphilis or tuberculosis,” Gurley said.

"People write about it and say it's very hard to do contact tracing because there's a pre-symptomatic, or asymptomatic, contagious phase," Sharfstein said. It's still possible to stop transmission if "you get to the contact before they are contagious. You just have to do it relatively quickly."

Before the pandemic, about 2,200 contact tracers worked in the country, according to the Association of State and Territorial Health Officials. Most states plan to expand their ranks of contact tracers dramatically. Few will need as many contact tracers as New York, which will supply the first wave of students to use the Johns Hopkins course.

In New York, the epicenter of the coronavirus outbreak in the United States, about 240 nurses and public health workers are working as contact tracers from a phone bank within the department of health, said Henry Garrido, president of District Council 37, the union representing health workers employed by the city.

On Friday, Mayor Bill de Blasio (D) told reporters the number of contact tracers in New York will swell by 1,000 by the end of May, as part of the city's Test and Trace Corps. More than 7,000 people have applied for positions through the Fund for Public Health, the nonprofit partner of the city's health department.

The mayor said hiring had begun, although a spokeswoman for the nonprofit declined to say how many people had been recruited. De Blasio said the city may need up to 10,000 contact tracers.

The city's public hospitals, not the health department, will take charge of these new tracers because the large hospital system is better equipped to organize a giant labor pool, de Blasio said Friday. That move has exacerbated tension between the city health department and the mayor's office, the New York Times reported Thursday.

The state health department expects to marshal 6,400 to 17,000 tracers statewide, agency spokesman Jonah Bruno said. The training of those workers will be funded in part by former New York mayor Mike Bloomberg, whose philanthropic organization donated $10.5 million. Interviews for those positions is underway, Bruno said. Bloomberg's donation also funded creation of the Johns Hopkins course.

The online course is a "key component of our program that will provide tracers with the tools to effectively trace COVID-19 cases at the scale we need to fight this pandemic," New York Gov. Andrew M. Cuomo (D) said in a statement.

Through it, the freshly recruited New York contact tracers will watch actors and Johns Hopkins public health experts, many of whom have worked as contact tracers, play out scenarios.

"This course gets into ethics, and it has role-plays, obstacles," Sharfstein said. For instance: If a contact tracer uncovers that an infected person is having an affair, should the tracer tell the family? (Answer: No.)

Completion of the course, marked by a certificate, is a first step, Sharfstein said, because professional contact tracers will require more specialized training to understand how the virus affects local communities.

Contact tracers are more than professional bearers of bad news. "This is about people working with humanity to stop the spread of the virus," Sharfstein said. "This isn't an activity of a cold, unfeeling bureaucracy. This is how we roll up our sleeves and protect each other."

Similar to social distancing, "contact tracing efforts are only going to be effective if communities buy in," Gurley said.

As they trace the virus, contact tracers are trained to provide support along with information. They can offer help with necessities such as laundry and meals, provide thermometers or masks, or help locate a safe place to isolate.

When asked about criticisms of contact tracing — that it is too slow or expensive to make a difference this late in the game, as argued by former New York lieutenant governor Betsy McCaughey in an April op-ed in the New York Post — Sharfstein said contact tracing was not a “magic solution.”

But "it was a critical piece of the puzzle for a number of countries that have been very successful in controlling the pandemic," he said. South Korea, Singapore and other Asian countries aggressively deployed contact tracing to curb the virus.

“I’m really not swayed by the argument that it’s too much to put these systems in place,” Gurley said. “And frankly we should have more of them anyway.”