(CNN) The organizers of the 2016 Olympic and Paralympic Games in Rio de Janeiro have said they will do everything possible to minimize risk caused by the Zika virus, which has been linked to thousands of birth defects.
As the World Health Organization (WHO) warned Thursday that there could be between three and four million Zika cases in the Americas over a 12-month period, Olympics officials outlined the measures being introduced to combat the threat.
The WHO warned that Zika was spreading "explosively" and said its estimate of cases covered all infections, including those of people who might not report or experience clinical symptoms.
Speaking to reporters in Switzerland, Dr. Sylvain Aldighieri said the lack of immunity to the virus and the fact the Aedes aegypti mosquitoes that spread it can be found almost "everywhere in the Americas" -- except for Canada and Chile -- "explains the speed" of its development.
The International Olympic Committee has predicted nearly half a million tourists will travel to Rio for the Olympic and Paralympic Games, with thousands of athletes also descending on the Brazilian city.
In a statement to CNN, Rio 2016 spokesman Phil Wilkinson responded to the latest developments by saying: "The Olympic and Paralympic venues will be inspected on a daily basis during the Rio 2016 Games to ensure that there are no puddles of stagnant water and therefore minimize the risk of coming into contact with mosquitoes.
"The Rio 2016 organizing committee is in regular contact with the Brazilian Ministry of Health and the Municipal Health department, which are the responsible authorities for guidance on health issues in Brazil and Rio.
"The organizing committee will follow the virus prevention and control measures provided by the authorities and provide relevant guidance to Games visitors."
The statement stressed that it was "important to note that the Rio 2016 Games will take place during the winter months of August and September when the dryer, cooler climate significantly reduces the presence of mosquitoes."
It said Games officials would "continue to monitor the issue closely."
Uriel Kitron, of the Department of Environmental Studies at Emory University in Atlanta, said: "I would say that there is probably at least a 50% reduction in transmission of dengue, for which we have long-term data and which is transmitted by the same mosquito vectors, in comparison to the peak months.
"Whether the Zika virus will behave exactly the same, we do not know -- but transmission will definitely be lower."
Professor Steve Lindsay, a public health entomologist at Durham University, said mosquito numbers "would be less in the cooler months" but warned: "Aedes aegypti bite during the day both inside and outside houses, so repellents should be worn during the day. So long as there are adults mosquitoes, there is a threat."
Earlier Thursday, the head of the WHO said anxiety over the spread of Zika was growing.
"The level of concern is high, as is the level of uncertainty," Director-General Dr. Margaret Chan told the organization's executive board members. "We need to get some answers quickly."
Chan said the virus was now in "23 countries and territories in the region," while the U.S. Centers for Disease Control and Prevention said it existed in 24 nations.
Zika has been around in some form for decades, but alarms have only recently been raised about its connection with "birth malformations and neurological symptoms."
Chan explained that where the virus has spread there has been a corresponding "steep increase in the birth of babies with abnormally small heads and in cases of Guillain-Barre syndrome."
Children born with a small head can suffer severe developmental issues, and it is sometimes a cause of death. Guillain-Barre is a rare autoimmune disorder that can lead to life-threatening paralysis, although most individuals fully recover from it.
After being first detected in a monkey in Uganda in 1947, Zika was most often found along the equator from Africa into Asia.
Nine years ago, new cases emerged in islands in the Pacific Ocean, and the virus -- for which there is no vaccine or cure -- made its way to the Americas last year.