It's not just humans who can travel rapidly across the world. Viruses and disease-spreading insects and animals can hitch a ride on various modes of travel and the ongoing Zika virus outbreak has spotlighted how viruses can span the globe, putting at risk people who had formerly been protected by geography.
The current viral outbreak was first reported in May 2015 in Brazil when the first confirmed cases were disclosed. But the cases quickly increased and now government officials believe that more than 1 million people have been infected in that country. World Health Organization officials said today that one reason for the rapid spread is that both people and mosquitoes have no immunity to the virus that is new to the region.
In a study published in June 2015 in the Memrias do Instituto Oswaldo Cruz medical journal, Brazilian researchers found that the genetic profile of the virus is linked to the Zika virus found in French Polynesia, a group of islands in the South Pacific.
This means the virus likely traveled to Brazil via an infected traveler who was then bitten by a mosquito, which then spread the virus.
"I think this provides pretty compelling evidence that it came [to Brazil] from French Polynesia. The main basis for that is the genetic similarity between the viral [samples]," said Dr. Peter Armbruster, professor of biology at Georgetown University, who was not involved in that study.
Prior to 2015, transmissions of the virus had not been found in people in South America and instead appeared mainly in Africa, Asia and Polynesia.
While it's not entirely clear when the viral outbreak started or if there was "patient zero" who spread the virus, researchers who published the June 2015 study speculated that the virus may have come to Brazil during major sporting events that brought together tens to hundreds of thousands of international travelers in close proximity.
"One plausible hypothesis is the arrival of the new emergent virus during the soccer World Cup in 2014," said the authors of the June 2015 study that studied the genetic sequence of the Zika virus in the Brazilian outbreak.
Another researcher in a paper for the U.S. Centers for Disease Control and Prevention's Emerging Infectious Diseases Journal pointed to the Va'a World Sprint Championship, a canoe race where four participating teams were from French Polynesia, as a possible place where the infection started.
Armbruster said the evidence is strong that it may have been an infected traveler from French Polynesia because the samples are from an Asian strain of the virus. However, he clarified that researchers cannot be certain it was related to the big sporting events.
"It is very likely an infected traveler from French Polynesia that traveled to Brazil was likely the source of the Brazilian invasion," he said. "Whether it is someone associated with the [World Cup,] we do not know for sure."
The current Zika virus outbreak in Brazil is a significant public health concern given the upcoming Olympics in Brazil, Armbruster said. If travelers to Brazil are infected and then bring the virus back to areas where the mosquito species Aedes aegypti is established, that could raise the possibility of local transmission of the virus.
The symptoms of the Zika virus are mild and include a flu-like illness with fever, rash, joint pain and red eyes. However, for pregnant women infected with this virus, it has been linked with babies that are born with small heads and underdeveloped brains -- a condition called microcephaly.
Officials are also investigating if the virus is associated with Guillain-Barr syndrome, which can cause muscle weakness and paralysis.
Dr. Bhavini Murthy is a preventive medicine physician at University of North Carolina-Chapel Hill and a resident in the ABC News Medical Unit.