Scientists are warning that the Zika virus is so dangerous, it would be unethical to allow the Rio Olympics to proceed as planned this August.

More than 100 doctors, researchers and health experts from around globe signed an open letter published Friday urging the United Nation’s World Health Organization to either move the summer games from Rio de Janeiro or to delay them, saying they are concerned about the virus’ potential impact on global health.

“The Brazilian strain of Zika virus harms health in ways that science has not observed before,” they wrote. “An unnecessary risk is posed when 500,000 foreign tourists from all countries attend the Games, potentially acquire that strain, and return home to places where it can become endemic. Should that happen to poor, as-yet unaffected places (e.g., most of South Asia and Africa) the suffering can be great. It is unethical to run the risk, just for Games that could proceed anyway, if postponed and/or moved.”

The letter listed seven key causes for concern, including Rio’s unsuccessful mosquito-killing program (Zika is spread by mosquitoes and by sexual intercourse) and a local health system that’s “so severely weakened as to make a last-minute push against Zika impossible.”

The Rio Olympics is scheduled to run August 5 to 21, and more than 500,000 foreign tourists are expected to attend the games.

Earlier this month, the WHO issued a statement warning Rio visitors about the Zika virus and encouraging them to follow protocol, such as protecting themselves from mosquito bites, practicing safer sex and staying in air-conditioned accommodations. The statement stopped short, however, of discouraging people from attendingor advising the Olympic Committee to rethink the event.