RIO DE JANEIRO -- The mayor of Rio de Janeiro said Thursday he doesn't believe the rapidly spreading Zika virus "is a problem for the Olympics," which open in six months in the picturesque South American city.

Eduardo Paes noted that the games starting Aug. 5 will occur in the drier, cooler South American winter season when controlling the mosquito population "will be much easier."

International Olympic Committee President Thomas Bach said Thursday his organization is in "close contact" with Brazilian authorities and the World Health Organization about Zika.

Bach said the IOC would send advice this week to all national Olympic committees, which can then tell athletes about safety guidelines.

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Brazilian President Dilma Rousseff 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 said 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."

The CDC has issued a travel alert advising pregnant women to postpone travel to nearly two dozen countries -- mostly in Central and South America and the Caribbean -- where Zika transmission is ongoing. CDC/CBS NEWS

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.

The CDC has issued a travel alert advising pregnant women to postpone travel to two dozen countries -- mostly in Central and South America and the Caribbean -- where Zika transmission is ongoing.

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.

Canadian Blood Services announced Thursday it will soon refuse blood donations from those who have travelled to countries where the virus has become widespread.

The World Health Organization estimated Thursday there could be three to four 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, a condition called microcephaly. WHO said there is an "extremely high" level of alarm that the virus could be causing a surge in the number of Brazilian babies being born with microcephaly.

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 -- qualifies as a global health emergency.