Zika began spreading in Florida mosquitoes about three months before infections showed up in the Miami area last summer, and the virus likely was carried in by travelers from the Caribbean, new research suggests.

Mosquitoes there started picking up the virus from infected travelers as early as March last year, according to scientists who examined genetic information from samples from about 30 people with Zika as well as from mosquitoes. It wasn't until July that Florida health officials said they had detected a local infection — the first in the U.S. mainland. Mosquitoes spread Zika by biting someone who's infected, then biting another person.

The bugs may have been causing infections in Miami as early as March, too, said researcher Kristian Andersen of the Scripps Research Institute in La Jolla, California. But there were likely few cases before July, and it's not clear any of them sought treatment, he said.

Most people infected with Zika don't get sick. It can cause a mild illness, with fever, rash and joint pain. But infection during pregnancy can lead to severe brain-related birth defects in babies.

Andersen said it likely took mosquitoes biting 30 to 40 infected travelers to produce the outbreak that flared last year in Florida. Most of the 256 cases reported in the Florida outbreak did not occur until late summer, he added. Health officials declared Miami-Dade County clear of continuing Zika infections by December, though isolated infections have continued, including this year.

Texas is the only other state that had homegrown Zika cases last year. All the other Zika cases in the U.S. have been connected to travel to areas with recent large outbreaks, mostly to South America and the Caribbean.

Zika that spread in Florida mosquitoes mainly came from the Caribbean, the genetic information studied indicated. About 3 million travelers arrived in Miami from the Caribbean during the first half of 2016. About 2.4 million of them came on cruise ships, but it's not clear that cruise ship passengers were the main spark in the Florida outbreak, Andersen said.

The Florida research was one of three papers on Zika published Wednesday in the journal Nature. The two others concluded there was a lag of six to 12 months between Zika's arrival and its detection in Brazil in 2015 and other parts of South America.

Screening efforts using new technologies — if developed further — could change that, wrote the University of Arizona's Michael Worobey, in an editorial accompanying the Zika articles.

"We should be detecting such outbreaks within days or weeks" and not months or years, he wrote.