On July 20, the Ebola outbreak that had been simmering in West Africa for months, growing more out of control by the day, arrived in Nigeria, Africa’s most populous country. It came in the form of a Liberian-American airline passenger named Jonathan Sawyer, who had left a treatment center in Monrovia, Liberia, against medical advice, and flown through Ghana and Togo to get to Lagos, Nigeria’s capital.

Lagos is a city of 21 million, yet the nightmare scenario of a fast-moving, out-of-control Ebola outbreak in a city with extensive transportation ties to the industrialized world never came to pass. In fact, Nigeria stopped the outbreak in its tracks, as did Senegal, which had a much smaller imported outbreak around the same time.

On Monday, the World Health Organization declared Nigeria “Ebola free.” (Senegal had also been declared Ebola free on Oct. 17, after seeing just one non-fatal case.)

What accounts for the successful control of the deadly virus in these two countries, compared to the failure of health authorities to successfully combat the epidemic in Guinea, Sierra Leone and Liberia? What lessons might Nigeria and Senegal hold for the U.S., which has seen its own Ebola scare in recent weeks?

In the hardest-hit countries, the death toll is rocketing toward 5,000, and total number of cases approaching 10,000, and the U.N. has warned that there could be 10,000 new cases per week by December.

Experts told Mashable that the lessons from the outbreak in Nigeria and Senegal can’t be readily applied to the poorer states at the center of this event, largely because of the disparity in available resources, but that they demonstrate to countries like the U.S. and Spain, which have had Ebola cases, that early intervention can contain the disease. In fact, Nigeria’s experience may offer some comfort to Americans who have been anxious in recent days about the virus’ spread.

One index patient leads to 18,500 in-person interviews

In all, Sawyer’s illness led to a chain of events in which 894 contacts were identified and followed for Ebola symptoms, amounting to a total of approximately 18,500 in-person interviews in Lagos, Port Harcourt, and other parts of Nigeria, according to a report from the U.S. Centers for Disease Control and Prevention (CDC). Yet even with this massive response effort, 19 confirmed cases occurred, along with one probable case, leaving 11 dead.

According to Adam Levine, an emergency medicine professor at Brown University who has treated Ebola patients in Liberia in recent months, the West African nations that have been the epicenter of this outbreak were caught off guard and ill-prepared. Their experience provided a warning sign for the Nigerian authorities, who then pounced when Sawyer’s illness was identified.

“The Nigerian government, seeing the epidemic growing in their neighbors over the past several months before the index case arrived in July, responded rapidly by setting up a thorough contact tracing system to identify and follow up all potential cases,” he said.

“Even so, 19 more people would fall ill from their single index case, but it could have been much worse had the Nigerian government not been prepared.”

Dr. Tim Flanigan, an infectious diseases specialist at Brown University who is currently in Monrovia, Liberia, fighting the outbreak there, told Mashable that the situation Nigeria faced was vastly different to what West African nations are dealing with. He compared Ebola to lightning striking a dry forest.

The virus, he said, creates “brush fires” of illness, that, if treated quickly enough, can be extinguished. If not, they grow into massive conflagrations, potentially engulfing whole forests. This is what is happening in West Africa, and the fate that was avoided in Nigeria, Senegal, and hopefully, the U.S.

“Nigeria was fortunate enough to trace Patrick Sawyer’s contacts,” Flanigan says, because that, in effect, helped put out the fire.

Experts say Senegal, too, attacked their much smaller Ebola outbreak in a similarly aggressive way, tracking down 74 contacts from the single patient and monitoring them for signs of the disease.

Levine said Nigeria’s example shows that the public health community does know how to stop Ebola. Now it’s a matter of scaling these efforts up.

“The tried and true methods used in Nigeria (and in all previous Ebola outbreaks) of isolation in treatment units, contact tracing, and community education can still work to stop this epidemic, but they need to be deployed on a massive scale several orders of magnitude larger than any previous effort. The question is whether the international community is up to the task,” Levine said.

This explains why the CDC has placed such a priority on contact tracing in the case of the Dallas, Texas, patients, for example.

The CDC studied Nigeria’s response to the outbreak to see if its efforts could be repeated elsewhere, concluding that there are many lessons for other African countries to learn from. However, once an outbreak is as out of control, as it is in countries like Liberia and Sierra Leone, it’s going to take a lot more than just a bureaucratic command post to put the demon back in the freezer.

“The Nigerians got on top of the outbreak straightaway, making sure that all contacts of Dr. Sawyer were identified,” Derek Gatherer, a virologist at Britain’s University of Lancaster, told Mashable in an email. “This resulted in 362 individuals being placed under surveillance. This was quite a close-quarters monitoring with travel away from Lagos prohibited –- a restriction on personal freedom which might be regarded as excessive in other countries.”

According to Gatherer, two people left observation, one of whom went on to develop Ebola and infect a doctor in Port Harcourt, Nigeria. The doctor then infected other patients. Yet in both Lagos and Port Harcourt, Gatherer said, “the Nigerians responded with energy and determination.

“Their advantages over the other countries are obviously wealth and size. Despite the extreme poverty found in many places, Nigeria has the resources to tackle something like this if there is a will within government. The second major advantage was that they knew what was happening at the earliest stages. The other three countries had little inkling of what was happening to them until it was rather too late to put the same measures into place, even assuming they had the resources (which they don’t).”

According to an analysis of the Nigerian outbreak published in the journal Eurosurveillance, a key factor in allowing Nigeria to prevent a broader Ebola outbreak was the quick identification of Mr. Sawyer, who was the first, or so-called “index case,” in that country.

“In contrast, the initial outbreak in Guinea remained undetected for several weeks. This detection delay facilitated the transnational spread of the virus ..,” the study states.

The study says the Nigerian example “offers a critically important lesson” to countries in Africa that have not yet been affected by the epidemic, as well as other nations including the U.S., where importation and spread of the disease is still a risk.

Adia Benton, an anthropology professor at Brown University, says part of Nigeria’s success came from the country’s use of social media.

“[The] Nigerians were very active on social media — especially Twitter — doing Ebola chats, and spreading the world amongst themselves.”

Benton says she is not entirely comfortable with the idea that Nigeria and Senegal have succeeded in their fight against Ebola, since new cases could arrive their from other countries at any time.

“… We know that they’re really never out of the woods as long as people can travel — and may even be seeking good care in these better-resourced places,” Benton said. “It seems that what we’ve learned is that early, pro-active identification of Ebola patients, their close contacts, and active monitoring of the sick and exposed are key to an outbreak response. The other three countries are well beyond this” point, she said, and far more resources and activities are necessary to bring the disease under control there.

As for West Africa, Flanigan, who is on the front lines in Liberia, said more resources are arriving, at long last.

“The resources and international aid now is dramatically improved from when I came in September,” Flanigan says. Although the international community is still “playing catchup” with the fast-moving disease, he said “The increase in support is clearly present.”