Borr was aware of Zika’s spread through the Americas, but the threat seemed distant. As a soon-to-be mother, she sympathized with the mothers of the infected babies in Brazil. But mosquitoes, though a nuisance, were simply nothing new.

“That’s just like a way of life in Miami, to be covered in mosquito bites,” Borr said. “You don’t think anything about it. You don’t realize that, in other countries, a mosquito bites you and you could be dead.”

Watching Zika more closely than the two women was a man named Chalmers Vasquez, operations manager of Miami-Dade County’s mosquito control team. He believed the region was ready to combat the mosquito that carried the virus. In the past, the department had handled such tasks with ease.

The county had recorded cases of dengue fever in 2010 and 2014, and chikungunya, also in 2014. Both viral diseases are transmitted by the same mosquito responsible for Zika: Aedes aegypti.

On July 15, 2016, Vasquez and some co-workers stopped for lunch in Fort Myers on their way back from a visit to another mosquito control district when his phone rang.

Zika had arrived in South Florida. It was the county’s first Zika case caused by local mosquitoes.

That night, he and his team set their first round of mosquito traps and began monitoring the population.

***

For more than half a century, Zika had kept a low profile.

Discovered in 1947 in a monkey from Uganda’s Zika Forest, the virus spread through mosquitoes in Africa and Asia. Humans rarely got the virus, and the symptoms were so mild that those who did rarely sought treatment.

Aedes aegypti mosquito 4 to 7 millimeters Diseases carried Yellow fever, dengue fever, chikungunya and Zika virus. Geographic distribution One of the most widespread mosquito species. Found in Africa and Asia and more recently reported in South America, Europe and the southern half of the United States. Transmission method Bites victims for blood meals. Small mosquito, large role in spread of disease Small in stature — just 4 to 7 millimeters in length — the mosquito Aedes aegypti plays a large role in disease as a carrier of yellow fever, dengue fever, Zika virus and chikungunya.



The mosquito’s crucial qualities include the ability to become infected and transmit viruses efficiently, says William K. Reisen, editor emeritus of the Journal of Medical Entomology. Most critical of all, though, Aedes aegypti seeks out and feeds on humans who are the only urban host for these diseases.



Because it breeds in water, Aedes aegypti is well suited to developing countries where many people are forced to store water, rather than receiving it direct from a faucet. Its eggs are also drought-resistant and can remain unhatched for 6 months or longer.



– Mark Johnson Source: World Health Organization

Aedes aegypti mosquito 4 to 7 millimeters Diseases carried Yellow fever, dengue fever, chikungunya and Zika virus. Geographic distribution One of the most widespread mosquito species. Found in Africa and Asia and more recently reported in South America, Europe and the southern half of the United States. Transmission method Bites victims for blood meals. Small mosquito, large role in spread of disease Small in stature — just 4 to 7 millimeters in length — the mosquito Aedes aegypti plays a large role in disease as a carrier of yellow fever, dengue fever, Zika virus and chikungunya.



The mosquito’s crucial qualities include the ability to become infected and transmit viruses efficiently, says William K. Reisen, editor emeritus of the Journal of Medical Entomology. Most critical of all, though, Aedes aegypti seeks out and feeds on humans who are the only urban host for these diseases.



Because it breeds in water, Aedes aegypti is well suited to developing countries where many people are forced to store water, rather than receiving it direct from a faucet. Its eggs are also drought-resistant and can remain unhatched for 6 months or longer.



– Mark Johnson Source: World Health Organization

In fact, Zika seemed the least troubling of the various diseases carried by Aedes aegypti. The others – Chikungunya, dengue fever and yellow fever – all continue to ravage countries that lack proper mosquito control.

By comparison, Zika had done little damage.

Before 2007, only 14 cases of Zika had been documented in people, according to the World Health Organization. Because only 20% of patients show symptoms, it can be difficult to track the movement of the virus.

Despite the scarcity of symptoms, blood tests showed the virus was actually very common in Africa and had spread to Southeast Asia: from 2% infected in North Vietnam to 75% in Malaysia.

Still, Zika wasn’t a high international concern until a small island in Micronesia, called Yap, began to notice a new illness causing rashes, pinkeye and joint pain. It looked like dengue, but something was different: the fever was milder and accompanied in some cases by pinkeye. In June 2007, lab samples sent to the CDC confirmed the disease was Zika. The stealth virus had caused its first outbreak.

In the end, an estimated 73% of Yap’s more than 7,000 residents were infected.

Soon, the virus began popping up in other places that had never seen it before – places where people had not developed immunity.

In May 2015, eight years after the outbreak on Yap, researchers in Brazil’s National Reference Laboratory confirmed Zika was circulating in their country. Reports of birth defects that would later be linked to Zika infection began to pour in: babies born with small heads and poor eyesight. The babies wailed in almost constant pain.

Zika’s sudden rise to international prominence continues to baffle experts. It is unclear how the virus went from anonymity to, as the World Health Organization called it, a “public health emergency of international concern.”

The global outbreak left scientists wondering how the severe effects of Zika, including the birth defects, went unnoticed for decades.

Some have suggested that in the new areas, there were simply more pregnant women vulnerable to infection. In Asia and Africa, where many had already caught the virus, the only people left to infect were children, and they would have immunity by the time they reached childbearing age, said Sharon Isern, a researcher at Florida Gulf Coast University.

Matt Aliota, a researcher at UW, suggested another possibility: The virus had never infected a population large enough to allow scientists to track the effects.

One other factor could be at play, according to a 2016 article in the journal Clinical and Translational Immunology by Isern and her colleagues: Dengue.

Lab work by Isern suggests a prior dengue infection could allow Zika to proliferate more in the body, causing the severe infection characterized by birth defects in babies. However, the theory remains controversial, and early research in primates at UW has shown there may be no such effect.

Brazil’s outbreak was curbed, thanks to a wide-ranging strategy of mosquito control – from fumigating 20 million homes, to releasing millions of genetically modified mosquitoes that produce infertile offspring. Even so, the outbreak crossed borders. As of March 2017, 84 countries have reported evidence of Zika in native mosquitoes.

The breeding habits and distribution of Aedes aegypti make it a beast to control, said Vasquez in Miami. Still, the mosquitoes themselves can’t travel very far – only a quarter mile in their lifetimes.

But people can. And so, Zika crossed the ocean and made its way to Miami.

***

In Kenya, the second week of January this year brought a fairly typical burden of disease: 61,000 new cases of malaria (30 years’ worth for the U.S.), and 3,200 cases of typhoid fever (six months’ worth for the U.S.).

On this particular week, though, the challenges facing Nairobi’s health care system were more of the slowly-ticking, time bomb sort: the kind illustrated by the gap between city laws and the lives of city residents such as Paul Mugai Ngage.

Deadly pandemic, fueled by war



The virus followed trade and shipping routes around the world but was aided by World War I, which was nearing its end. Within a day of the first known U.S. case, on an Army base in Kansas, hundreds of soldiers were ill.



As soldiers traveled throughout Europe, the virus went with them. The soldiers lived in tight quarters with poor sanitation,



Army doctors and nurses faced a gruesome progression of symptoms that left



All told, the virus killed between 50 and 100 million people worldwide and 675,000 in the U.S. according to the U.S. Centers for Disease Control and Prevention. That, by some estimates, is more than World War I and World War II combined.



Experts believe that a lack of immunity played a role in the flu killing many otherwise healthy young adults. Recent research suggests the 1918 strain of influenza was brand new, created from the combination of



Mutations like the one in 1918 are why scientists say the influenza virus is most likely to cause the next pandemic.



– Devi Shastri The Spanish Flu outbreak of 1918 was the worst pandemic in modern history.The virus followed trade and shipping routes around the world but was aided by World War I, which was nearing its end. Within a day of the first known U.S. case, on an Army base in Kansas, hundreds of soldiers were ill.As soldiers traveled throughout Europe, the virus went with them. The soldiers lived in tight quarters with poor sanitation, which may have helped the disease take hold Army doctors and nurses faced a gruesome progression of symptoms that left some patients dead in 24 hours or less , suffocated by blood and other body fluids collecting in their lungs. From September to November of that year, the virus infected 20 to 40% of U.S. Army and Navy personnel All told, the virus killed between 50 and 100 million people worldwide and 675,000 in the U.S. according to the U.S. Centers for Disease Control and Prevention. That, by some estimates, is more than World War I and World War II combined.Experts believe that a lack of immunity played a role in the flu killing many otherwise healthy young adults. Recent research suggests the 1918 strain of influenza was brand new, created from the combination of all four swine and human flu viruses and one other “extinct” strain Mutations like the one in 1918 are why scientists say the influenza virus is most likely to cause the next pandemic.– Devi Shastri

Deadly pandemic, fueled by war



The virus followed trade and shipping routes around the world but was aided by World War I, which was nearing its end. Within a day of the first known U.S. case, on an Army base in Kansas, hundreds of soldiers were ill.



As soldiers traveled throughout Europe, the virus went with them. The soldiers lived in tight quarters with poor sanitation,



Army doctors and nurses faced a gruesome progression of symptoms that left



All told, the virus killed between 50 and 100 million people worldwide and 675,000 in the U.S. according to the U.S. Centers for Disease Control and Prevention. That, by some estimates, is more than World War I and World War II combined.



Experts believe that a lack of immunity played a role in the flu killing many otherwise healthy young adults. Recent research suggests the 1918 strain of influenza was brand new, created from the combination of



Mutations like the one in 1918 are why scientists say the influenza virus is most likely to cause the next pandemic.



– Devi Shastri The Spanish Flu outbreak of 1918 was the worst pandemic in modern history.The virus followed trade and shipping routes around the world but was aided by World War I, which was nearing its end. Within a day of the first known U.S. case, on an Army base in Kansas, hundreds of soldiers were ill.As soldiers traveled throughout Europe, the virus went with them. The soldiers lived in tight quarters with poor sanitation, which may have helped the disease take hold Army doctors and nurses faced a gruesome progression of symptoms that left some patients dead in 24 hours or less , suffocated by blood and other body fluids collecting in their lungs. From September to November of that year, the virus infected 20 to 40% of U.S. Army and Navy personnel All told, the virus killed between 50 and 100 million people worldwide and 675,000 in the U.S. according to the U.S. Centers for Disease Control and Prevention. That, by some estimates, is more than World War I and World War II combined.Experts believe that a lack of immunity played a role in the flu killing many otherwise healthy young adults. Recent research suggests the 1918 strain of influenza was brand new, created from the combination of all four swine and human flu viruses and one other “extinct” strain Mutations like the one in 1918 are why scientists say the influenza virus is most likely to cause the next pandemic.– Devi Shastri

Ngage lives in a place that is absent from most maps, a blank space where more than 133,000 people live. Officially, the Kawangware slum does not exist. Neither does Korogocho, where the scavenged goods from the dump are sold. All of Nairobi’s slums are considered illegal settlements.

Yet there stood Ngage, smiling as he gestured across his yard.

“Welcome to my home,” he said, laughing and describing his age as “over 75.”

The little property where he lives belonged to his father, who now lies buried behind the house. Today, Ngage shares the modest concrete home – essentially one large open room — with his wife, six children and “many grandchildren.”

In Nairobi, keeping livestock in the city is illegal, as it is in many large cities around the world. The practice began falling into disfavor in the 19th century, considered unsanitary and a disease risk.

Yet there in Ngage’s yard roamed an assortment of goats, sheep, geese, chickens and the family dog. The jawbone of a dead cow lay in the dirt, just outside the pen where the family’s lone surviving cow was resting. Rabbits and pigeons stared out from a stack of hutches.

“What you should appreciate is that the goats, sheep, the geese, the chickens, all of them are in one area, where they interact,” explained Patrick Muinde, a veterinary research assistant at the Nairobi institute. “A disease which can be transmitted by geese can be passed to chickens with severe consequences.”

Ngage’s yard and the air around it amounted to a microbial stew, a single place in which different species deposited their wastes, bodily fluids, even the tiny droplets that spread influenza.

Many animals have their own strains of influenza.

When they are kept in close quarters, a single animal may be infected with two different strains. As a result, segments of genetic material from one strain can mix with those from the other to form something new, a process scientists call reassortment.