As the new coronavirus has proved capable of spreading between people and across borders, scientists have worked to crack the secrets of its ability to infect and kill.
Scholars from China and other parts of the world have put the virus under the microscope " it looks like an orb studded with spikes " and sequenced its DNA, hoping to find better treatment for those who contracted it and make vaccines to prevent infection.
But in their efforts to stop the epidemic's global transmission, public health researchers have so far been unable to answer one question: Who did the virus first infect?
The hunt for this person " also known as "patient zero" " could provide clues that help us contain the virus, which has sickened 83,000 people, mostly in China, and spread to more than 48 countries.
What is 'patient zero'?
The term can be used interchangeably with "index case" and generally refers to the first person infected by a communicable disease in an outbreak.
In the Covid-19 epidemic, more than one person can be considered patient zero, according to Sarah Borwein, a doctor specializing in infectious disease at Hong Kong's Central Health Medical Practice.
"For example, we know who patient zero is in the big cluster in South Korea, and that is helping to trace all the contacts and understand what happened," she said. "But we don't know who patient zero was in the Iran cluster, or the cluster in Italy."
South Korea has reported more than 2,000 confirmed Covid-19 cases and at least 13 fatalities. The biggest cluster of infections has been linked to a secretive Christian church in the southeastern city of Daegu.
At the center of the church cluster is a 61-year-old female member of the sect, who is South Korea's 31st case. Korea Centers for Disease Control and Prevention has described the outbreak there as a "super-spreading event."
Meanwhile, Chinese officials are still trying to trace the epidemic back to its source in China.
The first coronavirus case was reported to the WHO on December 31 and has been linked to Wuhan's Huanan Wholesale Seafood Market.
However, a new study published by a team of Chinese scientists has said the virus might have been imported from somewhere else.
The first known Covid-19 patient, a man who showed symptoms on December 8, had been discharged but said he was not at the Huanan market, the Wuhan government has said.
"We don't know who the very first patient zero was, presumably in Wuhan, and that leaves a lot of unanswered questions about how the outbreak started and how it initially spread," Borwein said.
Why is it important to find them?
Knowing who patient zero was would help prevent future outbreaks and provide information about how to prevent transmission, Borwein said. But as time passes, identifying the index case grows increasingly difficult.
"Figuring out who patient zero was wouldn't give us all the answers but it would help to map the path the virus has taken and how it's traveling," she said. "It's hard to draw that map without knowing where it starts."
John Nicholls, a University of Hong Kong clinical professor in pathology, said identifying patient zero during the severe acute respiratory syndrome (Sars) epidemic in 2002-03 was vital from an epidemiological perspective, as it highlighted the mode of its spread.
The disease, which infected over 8,000 and killed 813 people globally, was traced to a 64-year-old medical professor from Guangzhou, who had infected at least 13 tourists staying at the Metropole Hotel in Hong Kong.
But Nicholls, who was a member of the 2003 research team that isolated and characterized the Sars coronavirus, which is in the same family as the current virus, said the sheer number of infections in the current epidemic meant finding the patient zeros was a huge challenge.
"There are so many outbreaks and hotspots around the world, and this virus appears to spread more rapidly than Sars, so it's going to be a big challenge to accurately pinpoint patient zero," he said. "Epidemiological resources would be better at mitigating the spread rather than looking back."
Borwein, who worked as head of infection control for a hospital in Beijing and in outbreak communication during the Sars epidemic, said people must be wary of the language they use when talking about patient zero, because it had the potential to fuel xenophobia and panic.
"We talk about a 'desperate hunt' or a 'panicked hunt' for patient zero " and because coronavirus started in China, and patient zero was probably Chinese, it can easily fuel discrimination and racism," she said. "It's important to understand that patient zero is a clue, not a criminal."
What are some examples of Covid-19 patient zeros?
Most coronavirus patient zeros came from Wuhan in central China's Hubei province, the epicenter of the outbreak. That includes patient zeros in South Korea, Taiwan, Hong Kong, Macau, Singapore and the Philippines.
The first Covid-19 case outside China was reported on January 13 in Thailand. The patient was a 61-year-old Chinese woman from Wuhan who traveled to Bangkok with family members in a tour group.
Two days later, Japan confirmed the second coronavirus case outside China: a man in his 30s who lived in Kanagawa prefecture, southwest of Tokyo. The man had traveled to Wuhan and been in close contact with a pneumonia patient in the city.
On January 23, the US' Centers for Disease Control and Prevention reported the country's first American coronavirus case, a 35-year-old Washington state man who had visited family in Wuhan.
There have also been incidents of human-to-human transmission among Covid-19 patient zeros.
Vietnam confirmed two cases of the virus on January 23. A 65-year-old man from Wuhan met up with his son, 27, who works in the country and did not travel to China. The father developed a fever on January 17 and the son showed the same symptoms three days later, doctors said in The New England Journal of Medicine on January 28.
This family cluster suggested an instance of person-to-person transmission, the WHO said.
Germany also reported what is believed to be the first human-to-human transmission case in Europe on January 28.
A 33-year-old German man contracted the disease after attending a training session given by his Chinese colleague in the state of Bavaria.
The female colleague, who is from Shanghai, had recently returned from visiting her parents in Wuhan, said Andreas Zapf, head of the Bavarian State Office for Health and Food Safety.
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