The spread of the Zika virus is now being considered a "public health emergency of international concern" by the World Health Organization. The decision was made during an emergency committee meeting of independent experts that was held today. This is just the fourth time that the WHO has declared a public health emergency since the distinction was created in 2007.

The WHO issued the first public health emergency declaration in 2009 during the H1N1 swine flu pandemic. It issued two in 2014 — one for polio, and one for the Ebola outbreak. The WHO reserves this declaration for only the most severe cases for a number of reasons, especially because rarifying the distinction helps preserve its effectiveness. Declaring a public health emergency can severely impact the economies of affected countries, especially if travel or trade restrictions are put in place. It also puts the WHO in the place as the global coordinator over the crisis.

More broadly, it's meant to draw attention to the problem in hopes of increasing research, funding, and volunteer efforts. Those extra resources could help Brazil contain and manage the spread, but they may be even more critical to the 20 or more surrounding countries to which the virus has spread. Without intervention, the WHO estimates that Zika virus could infect as many as 4 million people in the region by the end of this year.

The Zika virus is transmitted by mosquitoes, and has infected 1.5 million people since it arrived in Brazil in May of 2015. Most people who become infected don't experience symptoms, and the 20 percent who do typically suffer mild symptoms like rashes, fever, and joint pain. But there has been a growing concern that the Zika virus is causing a condition called microcephaly in newborn children. Microcephaly causes abnormally small head sizes in these children, and stunts development of the brain.

Brazilian health authorities said last week that 4,180 cases of microcephaly have been reported since October, which would represent an incredible increase from the 150 cases typically reported per year. Another concern is that the Zika virus could be related to a spike in reported cases of Guillain-Barré syndrome, which can lead to partial paralysis. The CDC is currently investigating the link between Zika and these conditions while Brazil works to contain the spread of the virus.