The World Health Organization is not considering declaring an international health emergency because the organization still believes the hantavirus outbreak is contained and the overall public risk remains low, WHO official Maria Van Kerkhove said on Friday.

The WHO said there's no evidence that the virus changed to become more transmissible or more severe.

An international study involving around 20 countries is being launched to better understand how long people stay infectious and how long the virus can stay detectable in the body, according to the WHO.

The WHO is also working with colleagues "in Argentina, in Chile, in Uruguay and around the world" to understand the origin of the hantavirus outbreak, Kerkhove said.

Kerkhove said the teams in Argentina are looking at the movements of the first cases and past hantavirus outbreaks in South America, including where the virus was found in humans and rodents.

She said this is "pure field epidemiology" and that WHO hopes to report more “very, very soon.”

WHO is also tracing people and possible exposures connected to Saint Helena, Chile, Uruguay and South Africa as the organization tries to piece together the route of the outbreak.

-ABC News’ Dada Jovanovic

The American doctor who was on the ship and initially tested positive has since tested negative and says "there's no evidence that I've had hantavirus."

A doctor from the biocontainment unit in Nebraska said the initial test was most likely a falsely positive, based on further testing.

Dr. Stephen Kornfeld -- the only American to test positive for hantavirus -- came down with flu-like symptoms on the cruise ship, and on Monday he was admitted to the University of Nebraska Medical Center's biocontainment unit following a positive test, officials said. The Bend, Oregon, resident has since tested negative and was cleared to relocate to Nebraska's quarantine unit, where 15 other passengers from the cruise ship who do not have symptoms are being monitored.

"I physically feel great -- I have felt great for many, many days," Kornfeld told ABC News. "Emotionally I feel wonderful. It's nice to be negative for hantavirus."

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A team of Argentine scientists are planning to head to Ushuaia next week to conduct research to see if hantavirus was already there before the MV Hondius cruise ship departed on April 1, officials said.

The team will collect samples and send them to a lab. The results are expected to come back in about four weeks.

The World Health Organization has posited that the first infection likely happened on land before the ship even sailed. But local officials in Ushuaia say they are not convinced because they’ve never had a confirmed hantavirus case there.

“Its been 45 days since the ship left and we haven't had any cases,” said Juan Petrina, director of epidemiology for Tierra del Fuego province.

-ABC News' Dragana Jovanovic

Officials at Radboudumc hospital in the Netherlands say blood tests show that the 12 hospital employees placed in precautionary quarantine were not contagious to others.

The hospital said the chance of actual infection remains "very small," but the employees are continuing a six-week precautionary quarantine.

-ABC News’ Will Gretsky