Dr Sam Allen, an expert on infectious diseases, has warned all visitors to Rio could be in danger of contracting it.
A string of athletes - including golfer Rory McIlroy and tennis ace Milos Raonic - have pulled out of the Games due to the mosquito-borne virus.
It can cause birth defects in babies, including tots being born with tiny heads.
Dr Allen, 49, said: “The worry is that with the Rio Olympics less than a month away many athletes and travelling fans could be exposed and might carry this disease to other parts of the world.
“We are now seeing an unusually high number of babies being born with small heads in this region.
“It is now recognised as a serious complication of the Zika virus disease.
“And it is also reported that it can be transmitted from a carrier to a partner through sex.”
The first female-to-male transmission of the disease through unprotected sex was seen in the US last week. A woman in her 20s who returned home from a Zika-hit area infected her partner.
Researchers at the University of Sao Paulo claim the Olympics would result in no more than 16 additional cases of the disease.
But Olympic chiefs stressed the risk of catching the virus in August remains low.