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updated 10/16/2014 11:45:56 AM ET 2014-10-16T15:45:56

The current Ebola virus outbreak in West Africa has killed more than 4,000 people, but it started with one person: a 2-year-old child who died on Dec. 6, 2013.

It's unclear how the child caught the virus, but by Jan. 1, 2014, the child's mother, 3-year-old sister and grandmother had all died of the disease, according to a study published in the Oct. 9 issue of the New England Journal of Medicine. Their town in the Guéckédou region of Guinea sits at the intersection of three nations, giving the virus easy access to Liberia and Sierra Leone.

In a disease outbreak, it's important to find the first person to become infected with the pathogen — called "patient zero" — because knowing that person's history can help researchers determine how and when the outbreak started, said Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center in Nashville, Tennessee.

"From a medical and public health point of view, finding the first case is instructive because it first gives us a clue as to what the source was," said Schaffner, who was not involved in the research.

For instance, in the Guinea outbreak, the location of the first case also explained how the virus easily spread to three countries within a matter of months.

It's likely that Ebola is transmitted to humans when they hunt or eat infected bushmeat, Schaffner said. If a person contracts the Ebola virus after hunting for meat, epidemiologists might ask questions that could help them figure out how the virus jumped from animals to people.

In Guinea, a traditional healer who had contact with the 2-year-old got sick, and then gave the virus to a doctor, who, in turn, passed it on to his relatives, according to the New England Journal of Medicine study. The disease is not airborne, but spreads through contact with the bodily fluids, such as blood and vomit, of an infected individual. [ 2014 Ebola Outbreak: Full Coverage of the Viral Epidemic ]

In their study, the researchers mapped out the transmission of the virus, showing how it spread. First, the virus wound slowly through rural regions, and then spread rapidly as it moved into populated areas, they said.

Finding patient zero requires painstaking detective work.

"We call it shoe-leather epidemiology," Schaffner said. "Health care workers go out in the field and wear holes in their shoes, figuratively speaking, going from case to case."

It's like backtracking, said Lina Moses, an epidemiologist at Tulane University in New Orleans. "Every case has a point where [patients] can identify some type of exposure to someone who would be classified as a suspected case of Ebola," Moses said. "If you have the personnel and manpower to backtrack — if you have people who are still surviving — then you can trace it back."

When patient zero is identified, the next step is to figure out how the person got sick. "You'd like to know when this might have happened," Schaffner told Live Science. "What was the animal that you killed, or you found already dead? What activities did you do? Did you skin it in the field? Did you cut yourself while you were skinning? Exactly how exposed were you to the entrails or the blood of the animal?"

Tracking Ebola

After the 2-year-old's death in Guinea, the virus spread to Liberia in March, Sierra Leone in May and Nigeria by July. When researchers track the virus's transmission, it tells them how Ebola travels. For instance, in Guinea, the boy's family and caretakers caught it and passed it on to others. In Sierra Leone, the first cases arose after 13 women attended the funeral of a traditional healer who had treated Ebola patients in Guinea.

Tracking these routes of transmission "has been very illuminating, and just reinforces the notion that dealing with the sick directly, whether you're a family member or a traditional healer, are high-risk events for acquiring the infection and perpetuating the epidemic," Schaffner said.

The first case of Ebola in Nigeria started in Liberia. A man who had a fever and was under observation for the illness at a hospital in Monrovia decided to leave, even though doctors asked him to stay. On July 20, he flew by commercial airline from Liberia to Ghana, from Ghana to Togo and finally from Togo to Nigeria, according toan Oct. 3 report from the Centers for Disease Control and Prevention.

The man potentially exposed 72 people at the Nigerian airport and hospital, and died five days later, the report found. But Nigerian officials were largely able to contain the virus, by establishing an Ebola Incident Management Center supported by the state and federal Nigerian government in coordination with international partners. The center helped rapidly coordinate the response and take actions such as contacting people who had interacted with the man, isolating people with viral symptoms and decontaminating places the man had visited, the report found. As of Oct. 8, Nigeria had 20 cases of Ebola and eight deaths, the CDC reported. [ Where Did Ebola Come From? ]

Other countries have reported one or two cases. On Aug. 20, a 21-year-old man traveled by road from Guinea to visit relatives in Dakar, Senegal's capital and largest city, the World Health Organization (WHO) reported. He had a fever, diarrhea and vomiting, but doctors in Senegal initially diagnosed him with malaria. By Aug. 26, doctors referred him to infectious-disease experts, and he later recovered.

The United States has reported two cases, and Spain has reported one, in which a nurse caught it from a priest who was airlifted from Liberia and brought to Spain for treatment, the CDC reported. The Spanish priest, 75-year-old Miguel Pajares, died on Aug. 11.

The Democratic Republic of the Congo currently has a separate outbreak that is caused by another strain of the Ebola virus. Epidemiologists found patient zero in that outbreak was a pregnant woman who butchered a wild animal that her husband had killed and given to her for food. Ebola outbreaks are more common in the Democratic Republic of the Congo than they are in West Africa — this is the seventh to happen in the country since 1976, WHO reported

Detective work

In addition to the backtracking method, researchers can verify the virus's trajectory with genetic analysis. For example, in a recent study published in the journal Science, researchers reported that they sequenced the genomes of 99 Ebola viruses found in samples taken from 78 patients in Sierra Leone. 

They found that the virus was brought to West Africa within the past decade, likely by an animal. Moreover, the findings suggested the virus had made a single jump from an animal to a person, meaning that the outbreak was not being fueled by continuous exposure to infected animals.

"They discovered that [the genomes] were all tightly genetically related," Schaffner said, "indicating that there was a single introduction from the wild that then spread among the humans."

However, finding patient zero does little to help patients who already have the Ebola virus, Moses said.

Instead, trained health care workers are sorely needed to treat patients in West Africa, she said. 

Follow Laura Geggel on Twitter @LauraGeggel  andGoogle+. Follow Live Science@livescience,Facebook &Google+. Original article on Live Science.

© 2012 LiveScience.com. All rights reserved.

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