A little-known, untreatable virus is quickly spreading across the Americas

A little-known virus is gaining a major foothold in South and Central America.

The Zika virus, which has only recently moved beyond Africa and Southeast Asia, has already had debilitating effects in the Americas. It's especially problematic in Brazil, where it's appears to be connected to a serious birth defect.

The Zika virus was originally identified in 1947 in Uganda. It's primarily transmitted by tropical mosquitoes - the same kind known for spreading dengue - that pick up the virus from infected people, according to the CDC. It was relatively unknown until 2007, when there was an outbreak of the virus in Micronesia.

Until 2014, the virus had only broken out in Africa, Southeast Asia, and the Pacific Islands. That year, it spread to Easter Island and Chile. By May 2015 Zika had made its way to Brazil. In the past year, Brazil has seen have been more than 84,000 cases of the virus.

So far, there are no vaccines or treatments for Zika. The only way to prevent the infection is to avoid being bitten by mosquitoes.

Currently, nine countries in Central and South America have reported cases of the virus. Puerto Rico reported on December 31 that it had its first locally acquired case of the virus.

Once infected, only about 20% ever show symptoms, which are most commonly fever, rash, joint pain, and red eyes.

And, in the case of Brazil's outbreak, the virus appears to have a harmful effect on pregnant women. In total for 2015, thousands of babies - about 10 times the normal amount for a year - were born with microcephaly, a condition in which the brain is abnormally small. This birth defect was often found after the mother had Zika virus-like symptoms early in the pregnancy.

According to the Centers for Disease Control and Prevention, microcephaly can result from a number of different prenatal conditions, such as other infections, genetics, and exposure to toxic substances. Some of the babies recently born with microcephaly have tested positive for the Zika virus, but others have not, the CDC reports, making it difficult to draw a definite conclusion about the reason for the increased prevalence of the condition.

However, both public officials and independent scientists doubt the two instances are completely coincidental.

Several other viruses, including rubella and herpes, are known to cause congenital defects in babies born to mothers with the condition. And doctors have recently detected Zika RNA, the genetic material the virus uses to reproduce, in several fetuses born with microcephaly, Scott Weaver, the director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch, told the Genetic Expert News Service.

With this discovery, said Weaver, "The link between Zika virus infection and microcephaly has become reasonably strong."

Nevertheless, Weaver added that it's still a bit too early to rule out an alternative cause. To do that, scientists would need to link the mother's infection directly to the beginning of the child's microcephaly and prove that no other viruses were involved.

With the disease making it as far north as Puerto Rico, Zika has the potential to make its way into the southern parts of the US next. The New York Times reports this may have something to do with climate change increasing mosquito ranges. As climates shift and get warmer, mosquitoes have more area they can cover, which means the disease can spread farther north into places it might not ordinarily reach.

An epidemic won't be entirely because of a warming planet, but it could make things worse.