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During a trial for attempted murder in 1998, an analysis of the HIV virus was used as evidence — for the first time ever in the U.S. — to help prove a doctor's guilt: A jury found a Louisiana physician had injected his (former) lover with the virus.

Now over two decades later, virologists are scrutinizing strains of the new coronavirus in the same way, to trace where it came from. This modern genetic sleuthing, combined with more traditional methods of interviewing infected people, is invaluable for tracking how and where the virus is progressing. Scientists can often accurately trace a virus's journey.

"We can identify who the Typhoid Mary is," said Siobain Duffy, who researches the evolution of viruses at Rutgers University.

(Typhoid Mary references Mary Mallon, who infected many New Yorkers as an unwitting, "healthy carrier" of the infectious typhoid bacteria in the early 20th century).

Even before genetic testing is employed, epidemiologists, who track patterns of disease, can get a pretty good grip on where the virus came from. It's called "contact tracing," and it boils down to (safely) talking with infected people.

"It's done with syphilis and other diseases like measles," explained Elizabeth Marshall, an associate professor in the Department of Biostatistics and Epidemiology at the Rutgers School of Public Health.

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Local officials, with support from the Centers for Disease Control and Prevention (CDC), narrow down where the infected person has been and who they've been in contact with. Then, health authorities seek out those other contacts and follow the thread. "You keep going," explained Marshall.

Seeking out contacts is vital for this new coronavirus, which leads to the respiratory disease COVID-19 that results in more severe cases than the flu and is ten times more lethal. That's because around 80 percent of infected people have mild or no symptoms. They can walk around for five days before the first hint of infection hits. So it's important to find out who they may have been unknowingly infected while they spread the virus.

But, some people infected with coronavirus have no clue how they contracted the contagious bug. For example, they may wake up with a fever but hadn't traveled from an area rife with infections, like Italy, or on a particular cruise ship. They contracted it somewhere they live. These are unexplained cases.

Here, an analysis of the virus can show how the bug traveled to their community. It's hard evidence. "This is not new, untested science," said Duffy. "This is something trusted as fact in a courtroom."

The principle, called phylodynamics, is similar to building a species family tree, like one showing how humans are related to chimpanzees. Except this tree is rapidly changing, because one coronavirus can replicate 1 million times in just one human cell, resulting in inevitable changes and slight mutations.

"It's changing as it spreads," said Mark Cameron, an immunologist at Case Western Reserve University. "It's evolving."

And it's evolving "on fast forward," added Duffy.

These changes provide genetic fingerprints about where the virus came from. The virus that infected someone in China will evolve when it gets to New York City. So virologists can figure out what strain spread from what area, and build a tree.

It would be obvious, for example, if the strain you have came from someone in Boston, perhaps a cluster of people who had attended a conference — like the 70 infections that came from the Biogen meeting at the Boston Marriott Long Wharf hotel, where people picked food from buffets and shook countless hands.

(In the case of the attempted murder trial in 1998, a genetic expert concluded that the strain used to infect the victim was more similar to that of one of the doctor's HIV-positive patients than other HIV-positive people in the Louisiana region.)

There will always be holes in the coronavirus tree, at least temporarily, because of a glaring lack of testing in places like the U.S. — so people remain ignorant of their infection status. Fewer than 5,000 Americans had been tested as of March 9. Meanwhile, South Korea, is testing 10,000 citizens each day, and their infection numbers are plummeting.

This new coronavirus, which likely came from bats or pangolins in China, is already about 20 percent different than previous strains of coronavirus, explained Cameron. So virologists are looking for subtle changes within this new, already different strain.

How the virus changes isn't just important for tracing the outbreak. It's integral for making an effective vaccine. (It will take a least a year to make and test an FDA-approved vaccine) "If this coronavirus is changing, the vaccine has to change with it," Cameron said.

Because a vaccine is long-off (and medications to slow the virus down in sick people are at least some six months away), society is left with perhaps inconvenient and uncomfortable, but powerful, behavioral weapons to slow the coronavirus. Namely, experts call for social distancing. Until health authorities like the CDC can get a better grip on the outbreak, this means keeping at least three feet away from someone, or avoiding crowds.

"Social distancing goes a long way to prevent the rapid spread of the virus," Akiko Iwasaki, a professor of immunobiology at the Yale School of Medicine, told Mashable.

"It takes an unprecedented public health response to put a lid on this one," Cameron emphasized. 

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The good news is unprecedented things are starting to happen. The NBA shockingly suspended its season as its first player tested positive for coronavirus. Major universities across the nation have canceled in-person classes. Google asked tens of thousands of its employees to work from home.

Of course, not everyone can work from home. But at a time when social distancing is paramount, it's prudent, if not a show of societal altruism, to avoid crowds and large gatherings if you can. And in a web-connected world, many of us can.

This virus is and will likely be severe, if not deadly, for millions of people over 60 in the world. A leading Harvard epidemiologist told Science Vshe forecasts that 20 to 60 percent of adults worldwide will be infected (though some 80 percent of cases so far are mild, not severe).

And if you infect someone, there's a good chance you'll be traced, as seasoned experts follow the trail of the spreading pandemic.

"This virus has spread very easily worldwide," said Cameron.

UPDATE: March 13, 2020, 12:17 p.m. EDT This story originally called the epidemiological practice "contract tracing" instead of "contact tracing." The typo has been fixed.

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