The U.S. currently has no cases of the Andes hantavirus, officials with the Centers for Disease Control and Prevention said at a briefing Friday.

The Andes version of hantavirus does not pass easily between people, and requires close, prolonged exposure, said the CDC’s incident manager for hantavirus, Dr. David Fitter. The CDC only recommends testing symptomatic people, Fitter said.

He said anyone with high-risk exposure should follow strict quarantine protocols. He classified high-risk exposure as anyone who was still on the cruise ship between May 5 and May 10, as well as anyone who had close, prolonged contact with someone who has Andes hantavirus, including sitting on a plane near someone with hantavirus.

CDC officials said they will not offer any updates on the conditions of the 16 cruise ship passengers currently in quarantine in Nebraska, including if they are currently symptomatic, citing privacy.

-ABC News’ Eric Strauss

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

The MV Hondius left on its voyage April 1 from Ushuaia, Argentina.

On April 6, a 70-year-old Dutch passenger fell ill with fever, headache and diarrhea, according to the WHO. That passenger died on April 11.

The ship was between the British island territories of South Georgia and St. Helena in the middle of the South Atlantic, according to data from the ship tracking website MarineTraffic.

The ship sailed on for nearly two weeks, stopping near the island of Tristan da Cunha before reaching St. Helena, where the Dutch man's body was removed on April 24.

A total 29 passengers from 12 countries disembarked while the ship was in Saint Helena.

The Dutch victim's 69-year-old widow also disembarked and flew to South Africa, where she collapsed at an airport there. Two days later, she died from the disease, according to officials.

Another passenger aboard the ship, a German national, died on May 2nd, a day before the ship arrived in Cape Verde.

The next day, the WHO announced it was investigating a suspected hantavirus outbreak on the MV Hondius.

The ship is currently en route to the Canary Islands.