A woman looks at a Center for Disease Control (CDC) health advisory sign about the dangers of the Zika virus as she lines up for a security screening at Miami International Airport in Miami, Florida, U.S., May 23, 2016.[Photo/Agencies]

The WHO has advised that pregnant women avoid travel to Zika outbreak areas and that men who have been infected by or exposed to the virus practice safe sex, or abstain from sex, for up to six months.

Dr. Derek Gatherer, a lecturer in the Division of Biomedical and Life Sciences at Lancaster University, agrees that Zika is an international emergency because it's an "insect-borne, rubella-like agent" with no vaccine or treatment.

"But we need to put it into perspective. It is not sufficiently great an extra risk to justify cancelling the Games."

He said the concerns raised in the original letter and in a second document issued on Tuesday overstate the risks, in part because they assume that the virus circulating in Brazil is far different from earlier, African strains, and therefore, is far more dangerous.

"We have no evidence at all that Zika has functionally evolved," Gatherer said in an email.

He said the dramatic appearance of microcephaly in South America "is probably because the continent has never seen the virus before, there is no herd immunity and women of child-bearing age are being infected there."