The most recent case of sexual transmission involved a patient in Texas, U.S., who authorities confirmed yesterday had become infected with Zika following sexual intercourse with someone who had returned from a country where the virus is prevalent.

But two other cases of sexual transmission occurred in just the last eight years.

American doctor Brian Foy caught Zika during a trip to Senegal in 2008. After he returned to Colorado his wife contracted the virus despite not having traveled to Senegal, and there being no local threat of mosquitoes.

"Their children did not contract, thus ruling out general contact as a transmission route," said Dr Derek Gatherer, at Lancaster University.

"Sexual transmission was regarded as the most probable route."

The second case was found in 2013 in a man in Tahiti, from whose semen Zika was found.

Men in both cases reported genital pain during the infection.

The virus' route of sexual transmission and its as yet unproven but likely cause of abnormally small heads, or Microcephaly in babies, has Professor Edward Holmes of the University of Sydney scratching his head.

The virology expert is puzzled by how Zika seems to have changed little on a genetic level over the years and shifted from "not infecting lots of people and not causing Microcephaly, to where it is today where it is infecting many thousands of people and causing this very strange syndrome."

One explanation he offers for the occurrence of Microcephaly is that the virus might have jumped from jungle-dwelling mosquitoes to ones that prefer urban environments and human blood.

Still, he hasn't figured out why a mosquito-borne virus would cause Microcephaly.

"In my 25 years of virology I've never seen anything like it before," he said. "Maybe the virus has always had the propensity to cause Microcephaly. But we never saw it in sufficient numbers to be able to record those cases."

Zika has spread particularly rapidly across Latin American countries, with Brazil being the worst-affected country thus far and where 4,180 babies have reportedly been born with Microcephaly.