The letter was initially signed by 125 experts, but an additional 25 signed on later today.
"That while Zika’s risk to any single individual is low, the risk to a population is undeniably high. Currently, Brazil’s government reports 120,000 probable Zika cases, and 1,300 confirmed cases of microcephaly (with another 3,300 under investigation), which is above the historical level of microcephaly," the group said.
The group also claimed the WHO had a conflict of interest due to a decades-long partnership with the International Olympic Committee and said previous statements by WHO officials have been "troubling."
"To prejudge that 'there's not going to be a lot of problems,' before reviewing this evidence [on Zika virus effects] is extremely inappropriate of WHO, and suggests that a change in leadership may be required to restore WHO's credibility," the group wrote.
The WHO and the International Olympic Committee did not immediately respond to ABC News' requests for comment.
Art Caplan, director of the NYU Division of Medical Ethics and co-author of the letter, told ABC News that the group was not alleging any wrongdoing by the WHO or IOC but wanted to bring up these issues to spark a dialogue about the risks involved and encourage health officials unrelated to the Olympics to weigh in.
"What we’re really focused on is can we have transparent, open, frank, televised, out-in-the-open discussion with experts" unconnected to the Olympics, Caplan said. "We think WHO is close to the IOC. ... They work together a lot."
The big fear, Caplan said, is that the giant sporting event will enable the transmission of the virus through infected travelers to other parts of the globe that have yet to be affected by the disease.
"We’re worried about bringing the mosquito back to places it isn’t, like India," Caplan aid. "You have people who will be infected and ... there are people literally coming from everywhere."
Earlier this month, the director of the WHO addressed Zika virus fears amid the Olympics, saying the WHO would not call for the games to be moved but that they were using a "targeted approach" to decrease transmission and warning those most at risk not to visit the country.
"I do share the concern of some athletes and travelers and, as I said, it is very much an individual decision," Chan said at the time. "The role of WHO is to provide them with support so they can make the right decision."