The outbreak in Brazil has been underway since December, mostly in rural areas in the southeastern part of the country.
Yellow fever spreads to humans through the bite of an infected mosquito, and outbreaks usually occur in South America and Africa. There have been at least 326 confirmed cases, including 220 deaths, with hundreds of additional cases under investigation, according to the Pan American Health Organization.
But there are growing concerns the virus could spread to urban centers like Rio de Janeiro, Sao Paulo, Belo Horizonte and Vitoria - areas where tens of millions live and where the World Health Organization said the virus could likely start spreading in a human-to-human cycle via the Aedes aegypti mosquito.
Although there is a highly effective vaccine for yellow fever, it is not routinely given in Brazil's major urban centers, they said. With the recent outbreak, Brazilians health workers are now urging people to get vaccinated right away.
Zika, dengue and chikungunya viruses are transmitted by the Aedes or Haemagogus species and they move at a faster pace through mainly unvaccinated communities. Officials also said that they are anxious about infected people coming from the jungles of Brazil.
"In an era of frequent global travel, any marked increase in domestic cases in Brazil raises the possibility of travel-related cases and local transmission in regions where yellow fever is not endemic", Fauci and Paules wrote.
What started in December as a rural, sylviatic (jungle-related) yellow fever outbreak in Brazil is now pushing toward urban areas, which could pose a risk of worldwide spread to U.S. states and territories, similar to what happened with Zika virus, federal infectious disease experts warned yesterday.
"As with all potentially reemerging infectious diseases, public health awareness and preparedness are essential to prevent a resurgence of this historical threat", they write.
The majority of these are found in São Paulo, Minas Gerais, and Espírito Santo. The WHO list now includes northern Rio de Janeiro and all of Espirito Santo, according to WHO guidance updated this week.
Top infectious disease specialists are warning that a yellow fever outbreak in the jungles of Brazil could bring the disease to the United States. Most infectious disease clinicians haven't ever seen a case of yellow fever, he said.
"This isn't 'chicken little, the sky is falling, '" Dr. Fauci told the Washington Post. "It's a public health heads up".
Thanks to better mosquito control and better sanitation practices, yellow fever has been virtually eliminated in the USA and the Americas. During that outbreak, the world's emergency vaccine stockpile reserve was exhausted, making the outbreak more hard to control.
However, "early recognition may be hard in countries such as the United States, where most physicians have never seen a case of yellow fever", the experts pointed out.
The fearsome disease starts like a common flu, with symptoms of headache, fever, muscle pain, nausea and vomiting. While most of those infected experience no illness or mild illness, approximately 15 percent experience a more serious form of the disease.
In the Brazilian outbreak, 80 people have died from the 234 documented cases.