The Zika virus, a disease possibly linked to serious birth defects, was declared an international public health emergency by the World Health Organization (WHO) on Monday.

The virus is transmitted to humans through mosquitoes. It is related to dengue, yellow fever and West Nile virus. It was discovered in Zika, Uganda, in 1947, and remained in the Eastern Hemisphere (Africa and Asia in particular) until last May, when there was an outbreak in Brazil.

Until this past year, few people in the Western Hemisphere have been infected, so there is no built-up immunity. As a result, the virus has spread throughout the Western Hemisphere’s tropical region and, as of Tuesday, into the United States, infecting millions.

The virus does not cause problems for most. The majority of the people infected by the Zika virus show no symptoms and do not suffer lasting harm.

Scientists are concerned, however, that the virus is linked to the increase of infants being born with the condition microcephaly. Reports of infants born with microcephaly, which results in a smaller-than-normal head and often a damaged brain, have risen sharply in Brazil.

There is no treatment for children with microcephaly, as there is no treatment for an abnormally small head.

“There is no way to fix the problem, just therapies to deal with the downstream consequences,” Dr. Hannah M. Tully, a neurologist at Seattle Children’s Hospital who specializes in brain malformations, told the New York Times.

The exact connection between Zika and microcephaly is unclear, if there is a connection at all. Scientists have begun research on the effects of Zika on pregnant women in Brazil, Colombia and El Salvador to better understand that relationship.

The WHO decided that, given current information and current potential risk, declaring an international emergency was warranted.

“The evidence is growing and getting strong,” Dr. Margaret Chan, director general of the WHO, told the New York Times regarding the link between the Zika virus and microcephaly. “So I accepted, even on microcephaly alone, that it is sufficient to call an emergency. We need a coordinated international response.”

In response to the outbreak, the Center of Disease Control (CDC) has issued a travel advisory for people traveling to the Pacific Islands, the Caribbean, Central America, Mexico, South America and Cape Verde. All have reports of the Zika virus.

Even with the extra precautions, the United States has not been immune to the virus’ spread.  Travelers have carried the virus with them to the United States, and on February 2, the Dallas County Health and Human Services Department confirmed the first case of a local transmission.

This local transmission was unusual, as it was made through sexual contact between an infected person and a healthy person.

“This opens up a whole new range of prevention issues,” Dr. William Schaffner, chief of preventative medicine at Vanderbilt University Medical School, told the New York Times.

While concerning, it is unlikely Zika will spread far through sexual transmission. The CDC advises men having sex after traveling to regions where the virus is spreading to wear a condom.

Dr. Schaffner believes most prevention measures should focus on mosquito control.

“Mosquito transmission is the highway, whereas sexual transmission is the byway,” he told the New York Times. “Sexual transmission cannot account for this sudden and widespread transmission of the virus.”