If you want to know how quickly the Zika virus will spread, just look at what happened in Puerto Rico with a related virus, chikungunya, the head of the Centers for Disease Control and Prevention said Wednesday.
Puerto Rico can expect to be hit especially hard with Zika, because it's got the Aedes aegypti mosquito that carries and spreads both viruses, as well as dengue and yellow fever, CDC Director Dr. Tom Frieden said.
"We will likely see significant numbers of cases in Puerto Rico and other U.S. territories," Frieden told a House Foreign Affairs subcommittee hearing on Zika.
"It can spread very rapidly."
Chikungunya, like Zika, originated in Africa and spread very quickly once it got to the Americas. Like Zika, it's almost never deadly, but chikungunya causes racking pain, so doctors paid attention to its spread.
Related: What is Zika Virus?
"In May of 2014 the first chikungunya case was identified in Puerto Rico," Frieden told the hearing. He showed a series of slides documenting the spread of the virus across the Caribbean island and U.S. territory.
"By October it was in almost all of Puerto Rico."
CDC reports that by August 2014, more than 10,000 cases of chikungunya had been reported. Eventually, more than a quarter of the adult population was infected. The virus has spread in 45 countries or territories throughout the Americas with more than 1.7 million suspected cases reported to the Pan American Health.
Zika's spreading the same way. Frieden says he does not expect many cases of Zika virus in the continental U.S. because the right mosquito isn't common except in the far south and because people tend to live inside, with screens and air conditioning.
Zika causes milder illness than chikungunya but is worrying because it's strongly linked to a severe birth defect called microcephaly and perhaps to a paralyzing syndrome called Guillain-Barre. CDC has cautioned pregnant women to stay away from Zika zones if they can.