Summer’s nearly here, and this is, unfortunately, good news for the Zika virus. As a new study published in the journal eLife reveals, it is likely to spread through warm, humid nations just above and below the equator, putting up to 2.2 billion people at risk.

Although the virus is not particularly dangerous for most people, it has been conclusively shown to hinder the development of the brain of fetuses within pregnant women. In many cases, it appears to cause microcephaly – a condition wherein the brain is dramatically reduced in size. There has been at least one example where the virus has almost completely destroyed the entire brain of an unborn child.

Studies mapping the virus have already been published, but this is the first to take into account a range of environmental factors not previously considered in detail. The researchers also didn’t automatically assume that it would spread in the same way other diseases also transmitted by the Aedes aegypti mosquito already do during the summer months.

“Earlier maps were based on Zika being like dengue or chikungunya,” Dr. Oliver Brady, an epidemiologist at the University of Oxford, told BBC News. “We are the first to add the very precise geographic and environmental conditions data we have on Zika.”

^{Hot and humid environments not only allow the transmitting mosquito to thrive, but they also encourage the fastest replication rate for the virus itself. Messina et al./eLife}

One recent study made the not unreasonable suggestion that Zika will proliferate wherever Ae. aegypti can thrive. This particular mosquito is happy in hot, humid climates, which goes a long way to explaining why so much of South America has been showcasing record numbers of infections. This is epitomized by Brazil, where there have been at least 1.5 million cases to date, along with over 1,000 examples of microcephaly.

This study also predicted that it will spread further northwards as summer comes to the United States, and that cities like Miami and Houston will start showing an increase in Zika infections.

For their new study, the team of researchers didn’t just look at where the mosquito will thrive, but if the environmental conditions at breeding grounds are also beneficial to the virus itself. “It needs to be warm enough for Zika to replicate inside the mosquito and for there to be a large enough [human] population to transmit it,” Brady added.

^{The northern tips of Australia to the eastern Chinese seaboard are moderate- to high-risk regions. Messina et al./eLife}

Their new model shows that South America, particularly along the Amazon River and its tributaries, as well as along the coast, will almost certainly show a spike in cases during summer months. Large swaths of Asia and Africa will also become ideal replicating grounds for the virus.

This research also largely echoes the results of the aforementioned study focusing just on the U.S., in that Florida and Texas are particularly at risk. Conversely, Europe is seen as a low-risk territory.

^{Over 1,000 babies have been born with shrunken brains in Brazil alone during this particular outbreak of Zika. Mario Tama/Getty Images}

Summer has just ended in the Southern hemisphere, it’s on its way in the Northern, and nearly 31 percent of the human race is apparently at risk of contracting the virus. As the researchers note, there should have been a rise in cases already, but there hasn’t been – so what’s happening?

One possibility is that Zika is being misdiagnosed as another infection; another is that populations in high-risk areas, thanks to an already large number of cases, are becoming immune to the virus. Let’s hope it’s the latter rather than the former.