(CNN) The news out of Dallas in early February exploded around the world: Zika can be transmitted by sex.

Suddenly, a virus that wasn't supposed to be a game changer in the United States took on a whole new dimension. It spurred the Centers for Disease Control and Prevention to add recommendations for safe sex to the already growing list of precautions needed to protect pregnant women from getting Zika and possibly infecting their unborn babies.

But here's what is most startling: The Dallas case wasn't unique. The first known case of sexually acquired Zika in the United States actually occurred eight years ago in northern Colorado after a husband and wife reunion.

The story of how microbiologist Brian Foy obtained Zika in Africa and passed it to his wife, Joy Chilson Foy, when he returned home reads like a detective novel: frozen blood, false leads, a clever clue from Africa and finally success -- laboratory proof that Foy had given a mosquito-borne virus to his wife during sex.

Mosquito magnets

It was the summer of 2008. Colorado State University Professor Brian Foy was in southeastern Senegal with graduate student Kevin Kobylinski, collecting mosquitoes. They were studying how to stop the spread of malaria as part of a research grant from the Bill and Melinda Gates Foundation.

Graduate student Kevin Kobylinski, left, and microbiologist Brian Foy in Senegal.

Each day, Foy, Kobylinski and African researcher Massamba Sylla trekked miles from their camp to the villages of Ibel and Ndebou, nestled at the base of hillsides in this West African nation. They arrived before dawn so they could capture mosquitoes that had preyed on villagers in their homes during the night.

"One village was taking this drug, ivermectin, to cure themselves of a tropical parasitic worm," Foy explained to CNN's Chief Medical Correspondent Dr. Sanjay Gupta. "We were studying how well the drug affects the survival of the mosquitoes that transmit malaria."

It was hot, sweaty work -- just the thing to attract bloodthirsty mosquitoes looking for their next meal. But as not to scare them off, the men could not use any mosquito repellent.

Photos: First Zika transmitted via sex Photos: First Zika transmitted via sex Microbiologist Brian Foy poses on a hillside above the villages of Ibel and Ndebou in southeastern Senegal in the summer of 2008. Senegal is a small country in West Africa. Hide Caption 1 of 12 Photos: First Zika transmitted via sex From left: Graduate student Kevin Kobylinski, African researcher Massamba Sylla and CSU microbiologist Brian Foy pause during mosquito collection in Senegal. Hide Caption 2 of 12 Photos: First Zika transmitted via sex Brian Foy aspirates malaria-causing mosquitoes inside a villager's home in Ibel, Senegal. Only female Anopheles mosquitoes transmit malaria; they pick up a parasite called a plasmodium from infected people when they bite to obtain blood needed to nurture their eggs. Hide Caption 3 of 12 Photos: First Zika transmitted via sex Research assistant Alassane aspirates mosquitoes inside a villager's home. After feeding on humans during the night, the mosquitoes often hide in bedding and along ceiling rafters. Hide Caption 4 of 12 Photos: First Zika transmitted via sex A trap at the end of electronic aspirator shows collected mosquitoes. According to the World Health Organization, there were about 627,000 deaths from malaria in 2012; 90% were in sub-Saharan Africa. Hide Caption 5 of 12 Photos: First Zika transmitted via sex After collection in the trap of the electronic aspirator, the mosquitoes must be sucked out and transferred to specially prepared collection cups holding sugar water. Hide Caption 6 of 12 Photos: First Zika transmitted via sex African researcher Massamba Sylla, left, and graduate student Kevin Kobylinski visit a villager's home in Ibel, Senegal. Hide Caption 7 of 12 Photos: First Zika transmitted via sex The verdant village of Ndebou, in southeastern Senegal. The warm climate of Senegal makes it an ideal breeding environment for mosquitoes, particularly in the wet season. Hide Caption 8 of 12 Photos: First Zika transmitted via sex Villagers in Ndebou, Senegal, are part of a study on a parasite drug called ivermectin. The blood of villagers taking this medication seems to kill any mosquitoes that bite them. Hide Caption 9 of 12 Photos: First Zika transmitted via sex Kevin Kobylinski poses with village children. Hide Caption 10 of 12 Photos: First Zika transmitted via sex Kevin Kobylinski adds sugar water to collected mosquitoes. The research team compared the life span of mosquitoes from Ibel, where villagers where taking ivermectin, to mosquitoes collected from Ndebou, where villagers where not on the medication. Hide Caption 11 of 12 Photos: First Zika transmitted via sex From left: Kobylinski, Sylla and Foy dissect dead mosquitoes in their makeshift Senegal lab. Hide Caption 12 of 12

"As scientists, you tend to put your work ahead of your personal safety at times," Foy said. "We take malaria prophylaxis. I get all the vaccines that I can possibly get.

"But because of the nature of our work, tromping through the village until the sun rises, sweating way too hard, wearing aspirators on our backs that are kind of like big vacuum cleaners that suck the mosquitoes out of people's huts, unfortunately, we were bitten a lot."

Finishing up their work in late August, Foy and Kobylinski headed back to the United States.

"When we got back, I was fine," said Foy. "I was happy to see my wife, Joy, my four kids and everyone. It wasn't until approximately 10 days after we left the villages that I started feeling ill."

A puzzling illness

It began with a rash on his back and torso, headache, light sensitivity and ever-worsening joint pain and swelling.

"My wrists, ankles and my thumb joints in particular definitely got worse," Foy said. "I remember very specifically after a whole week had passed, it really hit me. I just needed to cover my eyes and lay down. I was really tired."

Foy thought he'd probably contracted a flavivirus, one of a family of mosquito and tick-borne arboviruses, transmitted by the female Aedes aegypti mosquito. She's an aggressive biter that only preys on humans and prefers to feed during the day, unlike the malaria-carrying mosquito, which bites at night. She also likes to live, breed and bite inside homes, rarely flying more than a short hop to her next human victim.

Old tires and other trash in Dakar, Senegal, are perfect breeding grounds for mosquitoes.

"We like to say she's the rat of the mosquito world, highly adapted to urban tropical environments," Foy said. "The species used to breed in small tree holes, in places like West Africa, but it seems to be a very simple leap for this mosquito to then lay its eggs in used tires, or cans and trash that are around human settlements."

"There were also lot of nonhuman primates in the villages in Senegal," Foy added. "Most of these arboviruses have their roots in primates. So mosquitoes might bite a monkey and pick up the virus and then bite the villagers, passing on things like dengue, chikungunya and even Zika."

But Zika, which was only discovered in a Uganda forest in the late 1940s, was the last suspect on Foy's mind. His bet was on dengue or chikungunya , two viruses characterized by fever, joint pain, swelling and rash. Despite his illness, the hunt was on.

"I'm a scientist. I want to figure out what's going on, and it's my field," Foy said. "I'm thinking, 'All right, I need to take my blood. If we can get blood while I'm having acute symptoms, then we have a chance of catching the virus. And I need to document my symptoms, and I need to call my student, Kevin, and see if he had the same thing.'"

Sure enough, Kobylinski had come down with a similar rash and body aches about the same time as Foy.

"There were definitely some times by the end of the day, I couldn't type at all," said Kobylinski. "It was difficult to operate things with my hands."

"I said 'All right, start documenting your symptoms, Kevin. We're going to get your blood drawn,' " Foy remembers. "We got our blood drawn, but then, of course, my wife got it.

"And that's what really started having the light bulbs go off in my head saying, OK, there's something odd going on here, because this should have been a mosquito-borne transmission."

The hunt for a mysterious virus

For Foy, the only scientific way to prove sexual transmission was to eliminate other potential causes, one by one. Since Chilson Foy had not left northern Colorado in over a year, if she'd been bitten by an infected mosquito, it had to be near their home.

"I grabbed some traps from my lab, and set them in my yard, to see if we had mosquitoes that potentially could be the same species that might transmit an arbovirus," said Foy. "But this was in early September in Colorado. There wasn't really anything out."

At any time of year, northern Colorado is not the place to find the Aedes aegypti in the United States. She's primarily a southern gal, with the biggest concentration found in the Florida Keys and along the Texas-Mexico border. The other, less efficient carrier of Zika is the Aedes albopictus, commonly known as the Asian tiger mosquito. She's also not likely to be found in Colorado.

The rash on Chilson Foy's back, documented as part of the research.

While Foy was mosquito hunting, Chilson Foy, a registered oncology nurse, was documenting her symptoms, taking snapshots of her rashes and getting her blood drawn and certified at a local laboratory.

"So, your wife has developed the same symptoms as you by now, and you're piecing this together. How sick does Joy get?" Gupta asked.

"She got worse than I did, by far," Foy said. "Her joint pain was stronger, and it lasted a lot longer. She couldn't really open cans, and things like that, for quite a long time. And I think her symptoms of headache and aversion to light were a lot stronger than mine."

Another clue emerged. As Foy's illness progressed, he began having prostate pain and blood in his semen, mounting evidence for a sexual link to the virus transmission.

"But when you gave the virus to your wife, you were feeling fine, right?" asked Gupta. "You weren't sick at that point?"

"Exactly," Foy said. "We had just seen each other for the first time in months. You do what husbands and wives do. But later, when I'm feeling sick, I'm not having any sort of relations. I think any transmission event would have happened very soon after I got back, before I got symptoms."

'Many things we don't yet know about Zika'

Speaking in front of Congress last week, CDC director Dr. Tom Frieden was blunt.

"We are figuring out more about Zika literally every day," he said. "But as hard as we work, there are still many things that we cannot know now, and cannot do now."

With the current state of knowledge about Zika, health officials believe that a person is only contagious for as long as they are symptomatic. The problem is that only one out of five people with Zika will have any symptoms at all. In Foy's case, it would appear that he was contagious before the worst of his symptoms began.

And then there's the issue of how long the virus will last in semen.

Male testicles, along with the eyes, placenta and the fetus, are what is known as "immune privileged" sites, areas of the body that have special protection from an inflammatory immune response. Simply put, it means your antibodies aren't allowed in to kill off invaders, an evolutionary way of protecting procreation.

"Immune privilege" affected survivors of the most recent Ebola outbreak.

One U.S. man discovered Ebola in his eye months after his blood was declared Ebola-free. And Ebola has been found in the semen of male survivors in Sierra Leone for up to nine months after it had disappeared from their blood, although the volume of the virus does appear to reduce over time.

Other than the Dallas case, scientists today know of two more cases where Zika virus has been found in semen. One was during a 2013 outbreak in French Polynesia, just as the virus began its explosive spread around the world. A Tahitian man had three separate episodes of Zika; after the third, his semen showed high levels of the virus , even though it was undetectable in his blood.

second documented case in the United Kingdom in 2014 found high levels of the Zika virus in semen up to 62 days after the onset of the illness; in fact the viral load was stronger at that time than when the first samples were taken.

But in 2008, when Foy was searching for answers, those clues weren't available.

No sign of dengue or yellow fever

By now, the blood test results had come back from the Centers for Disease Control's Division of Vector-Borne Diseases in Fort Collins, Colorado, not far from Foy's lab at the university. Foy and Kobylinski's antibody panels showed a cross-reactivity to dengue and yellow fever, meaning they had been vaccinated or exposed to those viruses.

But Chilson Foy's results showed nothing. No dengue. No yellow fever. No antibodies to any of the viruses on the panel.

Zika was not on the test. It was too new, too isolated, too obscure. No one had thought of adding it.

Puzzled, Foy tossed the blood samples back into the fridge at his lab and went on with his life.

A case of serendipity

Fast forward a year. Graduate student Kobylinski was back in Senegal, replicating malaria field studies in additional villages.

In a twist of fate, he found himself having lunch with Andrew Haddow, a researcher working for the U.S. Army Medical Research Institute of Infectious Diseases. Haddow's grandfather, Alexander Haddow, was a famous entomologist who had been studying tropical diseases in the Ugandan forest for decades.

"We were drinking beers and discussing the difficulties in working in Africa with no electricity or running water," Kobylinski told CNN's Gupta. "His research team was there to study obscure viruses that affect people in that region. I mentioned that we probably had one of these lesser-known viruses and described the results that we had received from the CDC and the symptoms that we had. He told me he thought it could potentially be Zika virus."

"How did his mind go to Zika?" asked Gupta. "Why did he think that?"

"Well, his grandfather was the man who discovered Zika virus," said Kobylinski. "You know, out of the Zika forest in Uganda."

Back in the states, Foy was intrigued.

"We basically decided, 'Jeez, we should research this,' because we weren't really satisfied with the original results; they were inconclusive," remembers Foy. "Especially with relation to my wife."

So out came the frozen blood samples. This time, they sent them to Haddow who had the blood processed at the University of Texas Medical Branch in Galveston, Texas. The tests returned as positive for all three samples, including Chilson Foy's: It was Zika.

Another lab confirms the results. And finally the mystery was solved, at least to the extent that it could be. The scientist in Foy was still cautious about interpreting his findings.

"We didn't actually isolate the virus. We weren't able to. We weren't able to catch it in time," he said. "Our data was indirect confirmation of that hypothesis through antibody tests. After we followed the antibodies, I clearly had Zika. Kevin clearly had Zika, and my wife definitely had Zika, because she didn't have any cross reactive antibodies to anything."

Excited, the team decided to publish what they'd found. Carefully titled "Probable Non-Vector-borne Transmission of Zika Virus," the paper came out in May 2011 in the Journal of Emerging Infectious Diseases. While Foy, his wife and Kobylinski are listed as authors, their identities are disguised as Patient One, Patient Two and Patient Three to protect their privacy, something extremely important to Chilson Foy. However, it wasn't long before Foy, never one to dissemble, was outed.

"Once a clever reporter read that paper, they kind of instantly put two and two together, and said, 'It's clear that you're either Patient One or Two, and the other one is Patient Three,'" said Foy. "I admitted as much, and of course, after that, it kind of went crazy."

But not in a good way. The media invaded Chilson Foy's privacy at every turn, and scientific mentions of the paper often dismissed the findings as "inconclusive." "unsubstantiated" and a "one-off case."

That is until now. Until Dallas.

"Is it redemptive, in a way?" Gupta asked Foy. "Nobody wants to be the first when it comes to something like this, but that you recognized it, you wrote it, you did all this almost a decade ago?"

"I kind of feel proud about that," Foy said with a smile. "I think anybody who was skeptical about it, any colleagues, they were just being skeptical scientists, as they should have been. They wanted to see more evidence."

The smile fades. "Now, unfortunately, we have more evidence."

Now Foy isn't thinking about his scientific detective story. Instead he's thinking about what could have been, if things had been different.

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"The real tragedy of all this is that right after we wrote that paper, we wrote a grant and submitted it to the National Institutes of Health to try to get funding to research this further," Foy said, "with the hypothesis that Zika virus outbreaks are only going to happen more frequently, and we think sexual transmission is a possibility."

The grant didn't get funded. Foy said there's just not enough money to fund "high-risk, high-reward, hypothesis-driven projects," especially when the paper seems like a "one-off thing."

"If we would have had that work ongoing, way back when, we would have had the ability to research it, to maybe even predict that sexual transmission was going to be more frequent, when a potential virus outbreak occurred," Foy said. "Maybe we would've even been on the road to therapeutics or vaccines, but now we're kind of behind, and trying to catch up."

Never one to quit when it comes to science, Foy resubmitted the 2011 grant to the NIH on Tuesday. He should hear if it's being considered in about three months.