Four countries face the highest risk of a Zika outbreak if a single one of their athletes or travelers becomes infected during the Summer Olympics in Rio de Janeiro, according to a risk analysis published Wednesday by U.S. health officials.

The four countries -- Chad, Djibouti, Eritrea and Yemen -- have the factors that could result in a sustained spread of the mosquito-borne virus in a worst-case scenario, according to the Centers for Disease Control and Prevention.

"Except for these four countries, the 2016 Olympic and Paralympic Games do not pose a unique or significant risk for further mosquito-borne transmissions of Zika virus in excess of that posed by non-Olympic travel," the report said.

Brazil, which is hosting the Olympics, which begin Aug. 5, and the Paralympics, which open Sept. 7, is at the epicenter of the epidemic that has spread to nearly 50 countries, most of them in Latin America and the Caribbean.

The CDC conducted the analysis to predict those countries at risk for importing Zika virus solely from "introduction by a single traveler" to the competitions. As many as 500,000 international visitors and athletes from 206 countries are expected to travel to Rio for the Olympic Games and Paralympic Games, according to Brazilian tourism officials.

But overall travel volume to the Olympics represents a very small fraction -- less than .25 percent -- of the total estimated 2015 travel volume to Zika-affected countries, the analysis said, highlighting "the unlikely scenario that Zika importation would be solely attributable to travel to the games."

"The real bottom line message here is that globalization is the main driver for the spread of emerging infectious viruses, including Zika, and the relative contribution of the Olympic Games in Rio during their winter of 2016 is a proportionately very, very small part of that risk," said Martin Cetron, director of the CDC’s Division of Global Migration and Quarantine, who oversaw the analysis.

The CDC based its assessment on several risk factors: evidence of ongoing or previous Zika virus transmission; the presence of the Aedes aegypti mosquito, the primary vector; climate conditions in August and September; the potential for epidemics of dengue, a related virus that is often used as a proxy for Zika; and the absence of a history of Zika virus.

Of the participating countries, 19 had all of the above criteria. But to figure out the likelihood that the spread could be the result of travel to the competitions alone, researchers also ranked countries by the estimated air travel during the Olympics and otherwise.

For 15 of the countries, travel to Rio de Janeiro during the Olympics is not estimated "to increase substantially the level of risk above that" incurred by the usual air travel for these countries, the report said.

But the four countries "have an additional, unique factor: Apart from projected travel to the games, these countries do not have substantial travel to any country with local Zika virus transmission," the report said.

The four countries are estimated to have a total of about 60 people -- athletes and others -- attending the competitions.

The CDC made several assumptions to create its model for worst-case scenarios. Researchers assumed that Zika transmission would be ongoing during the winter months of August and September in Rio de Janeiro, typically a low season for mosquito-borne disease transmission. They also assumed that many visitors would not take preventive measures, such as wearing long sleeves and pants or using insect repellent, or that those would be ineffective, and that all visitors would have an equal risk for exposure to the virus.

They also assumed that visitors who were infected would return immediately to their home countries with no extended stays elsewhere, that they'd have enough virus in their system to spread the disease once they got home, and that the travelers wouldn't take precautions to prevent mosquito bites at home.

Cetron said the CDC has shared the risk assessment with the World Health Organization, which is working with the four countries to emphasize the importance of prevention measures and safe sex practices. Zika is transmitted primarily through the bite of an infected mosquito but can also be spread through sex.

The CDC is also providing Zika diagnostic kits to countries to help them test people for the virus. More than 100 countries now have the kits, but it's not clear whether these four nations have received them, Cetron said.

"This is an ongoing process to focus on the gap areas," he said. "We're still in the scale-up phase. Even if the individual country doesn't have them, many are part of regional lab networks where they can send a specimen and get a test result."

As the little-known disease's risks have become more understood, some experts have called for the Olympic Games and Paralympic Games to be moved or postponed because of health concerns. Nearly a dozen athletes, many of them golfers, have said they aren't going to compete in Rio because of concerns about Zika.

There is no treatment or vaccine for Zika virus disease.  About 80 percent of people who get infected have no symptoms, while the rest tend to only have mild symptoms that last for several days to a week.

In rare cases, though, the virus has been linked to a nervous system disorder that can cause muscle weakness and sometimes paralysis. It poses its biggest danger during pregnancy, when infection can cause a range of severe fetal abnormalities, including the condition known as microcephaly, where the brain is often underdeveloped. Zika has also been linked to an increasing number of cases of Guillain-Barré syndrome, which causes muscle weakness and, sometimes, paralysis.

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