Health officials say the number of U.S. residents diagnosed with Zika infections in the past year has grown to 31.
All of them are believed to have caught the infection while traveling in the Caribbean or Latin America where there are outbreaks of the tropical illness.
Officials said Thursday the 31 people are in 11 states and Washington. In U.S. territories, Puerto Rico has 19 confirmed cases and the U.S. Virgin Islands has one.
The government is looking at the issue of blood donations from travelers, although officials think the virus is gone from an infected person's blood in a week or less.
Brazilian President Dilma Rousseff has called for a meeting of the member nations of South America's Mercosur trading bloc to discuss ways to join forces to eliminate the Aedes mosquito and the Zika virus it transmits.
The Brazilian presidency's website says she told reporters covering the Wednesday summit meeting of the Community of Latin American and Caribbean States in Quito, Ecuador, that the Mercosur meeting will be held Feb. 2 in Montevideo, Uruguay.
"We must declare war on the mosquito and until we have a vaccine against the Zika virus, that war must focus our efforts on eliminating its breeding grounds," Rousseff said on her Twitter page.
"Eliminating Zika is everyone's responsibility. We must eradicate all areas of stagnant water where the mosquito lives and reproduces."
International Olympic Committee President Thomas Bach says the fact that the Olympic Games in Rio de Janeiro will be held in August -- during Brazil's winter -- could limit difficulties caused by the outbreak of the mosquito-borne Zika virus.
Bach spoke during a visit to Greece where he promoted a $2 million international assistance program for refugee athletes.
"The Olympic games will take place during the Brazilian winter so at that time we will have different climate conditions than there are now, when we are in the middle of the Brazilian summer," he said.
Bach said the IOC is in "close cooperation" with Brazilian authorities and the World Health Organization, and he promised to have consultations with national Olympic committees on safety guidelines concerning the Zika outbreak by late Friday.
The World Health Organization says China and other countries with dengue fever should be on the lookout for Zika virus infections.
Dr. Bruce Aylward, who runs WHO's outbreak response department, said any country that has the Aedes mosquito should be concerned about the possibility of the Zika virus arriving.
The Aedes mosquito spreads diseases including Zika, dengue fever and chikungunya. During a special session on Thursday, WHO said it is convening an expert group on Monday to advise on whether the Zika outbreak --which has now spread to more than 20 countries in the Americas -- qualifies as a global health emergency.
WHO says there is an "extremely high" level of alarm that the virus could be causing a surge in the number of babies being born with abnormally small heads in Brazil.
The World Health Organization estimates there could be 3 to 4 million cases of Zika in the Americas over the next year.
Sylvain Aldighieri, head of WHO's epidemic response team in the Americas, said the estimate is based on previous numbers of infections of dengue fever, which is also carried by mosquitoes.
He said the agency expects "huge numbers" of infections because of the widespread presence of the mosquitoes that spread Zika and because there is no immunity among the population.
He said that since most people with Zika don't get sick, there is a "silent circulation" of the disease that may make tracking its spread more difficult
Zika is suspected of being behind the birth of babies with abnormally small heads.
The World Health Organization has called Thursday's special session on Zika in part to convey its concern about an otherwise mild illness that has sown fear among many would-be mothers in Brazil, who have often responded by covering themselves head-to-toe in clothing in the often hot, largely tropical country or slopped on many coats of insect repellent.
"The possible links, only recently suspected, have rapidly changed the risk profile of Zika from a mild threat to one of alarming proportions," said WHO Director-General Dr. Margaret Chan.
"The increased incidence of microcephaly is particularly alarming, as it places a heart-breaking burden on families and communities."
The U.S. Centers for Disease Control says the disease is now in more than 20 countries, mostly in Central and South America.
A U.S. health official says he doubts the United States is vulnerable to a widespread outbreak of a virus linked to a wave of birth defects in Brazil.
Dr. Anthony Fauci says the Zika virus -- suspected of being connected to microcephaly -- hopefully can be kept at bay with "mosquito vector control."
Fauci, who heads the National Institute of Allergy and Infectious Disease, appeared on "CBS This Morning." He said administration officials do not believe there are major ways of spreading the virus "other than by mosquito bites."
Health officials suspect that Zika is linked to a wave of birth defects in Brazil in which babies have small heads.
President Barack Obama hosted a meeting of federal health specialists on the issue earlier this week.
The World Health Organization says it is convening an emergency committee on Monday to decide if the Zika virus outbreak should be declared an international health emergency.
At a special meeting on Thursday, WHO Director-General Dr. Margaret Chan said the virus -- which has been linked to birth defects and neurological problems -- was "spreading explosively." Chan said although there was no definitive proof that Zika was responsible for a spike in the number of babies being born with abnormally small heads in Brazil, "the level of alarm is extremely high."
WHO last declared an international emergency over the devastating 2014 Ebola outbreak in West Africa, which ended up killing more than 11,000 people.
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