Here's why the US Zika outbreak could be more widespread than you think

The Zika virus spent the past year spreading through Latin America and the Caribbean. And, just as experts had warned, it's now reached the continental US, with confirmed local transmissions of the virus in Miami, Florida.

The virus is transmitted by the Aedes aegypti mosquitoes, but can also be spread through sexual contact from an infected person to his or her partner. However, only 20% of people infected with Zika exhibit any symptoms - such as a fever, rash, bloodshot eyes and muscle aches - so the virus largely goes undetected.

Testing for the virus is also a complicated, expensive, time-consuming and not always accurate process, according to The Guardian. As a result, infected people might not actually be getting tested for it, and the virus could therefore be circulating in other US states undetected.

"There is not active surveillance going on in the at-risk states in the United States," Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Texas, told The Guardian. "I think there's not just Zika transmission going on in Miami, it's going on all up and down the Gulf Coast and in Arizona, it's just that nobody's looking."

In addition to Florida, other states in the Gulf Coast are believed to be at-risk for local Zika transmission. Hotez explained to INSIDER in July that urban neighborhoods in Texas, Louisiana and Florida, in particular, pose the highest risk.

"From everything we've seen with Zika, it seems to be the poorer areas that are the most at-risk," Hotez told INSIDER. "These are the mosquitoes that live in old discarded TV sets, plastic containers and tires."

Aedes aegypti mosquitoes are endemic, and travelers have returned from Zika-affected areas with the infection, increasing the risk of transmission in those areas, the Guardian noted. In fact, in Louisiana alone, there have already been 22 cases of travel-related Zika infections confirmed by the Centers for Disease Control and Prevention.

Undetected Zika outbreaks are particularly troublesome, because of the birth defects that the virus can cause. It's already been confirmed that Zika can cause microcephaly, in which babies have abnormally small heads and underdeveloped brains, while a new study suggests it may cause severe joint deformities, as well.

"The big fear, of course, is we'll figure this out seven, eight, nine months from now, in the spring of 2017, when we start seeing babies show up with microcephaly," Hotez told the Guardian.