The World Health Organization (WHO) has identified Pakistan as a country that has allowed polio to spread beyond its borders, contributing to what has now been declared an international public health emergency. Bloomberg Businessweek reports that Pakistan will now need to meet international travel restrictions, requiring citizens to present proof of a polio vaccination before they can travel internationally.

According to the WHO, polio (short for poliomyelitis) is a highly infectious viral disease, which mainly affects young children. The virus is transmitted through contaminated food and water, and multiplies in the intestine, from where it can invade the nervous system. Many infected people have no symptoms, but do excrete the virus in their feces, hence transmitting infection to others.

Polio symptoms include fever, fatigue, headache, vomiting, stiffness in the neck, and pain in the limbs. In a small proportion of cases, the disease causes paralysis, which is often permanent. Polio can only be prevented by immunization.

While polio was once seen around the world, it has largely been eradicated and CNN reports that it is endemic to only Afghanistan, Nigeria, and Pakistan. Earlier this year, the public health community certified Southeast Asia as polio-free. That included India, but not Pakistan, which accounted for 80 percent of polio cases this year. In fact, the second-highest national count for polio was four in Afghanistan, and those cases were linked to viruses that originated in Pakistan.

There is promise for eradicating polio in Pakistan. Once considered the hardest place to end polio, India boosted disease surveillance and immunization efforts to vaccinate hard-to-reach communities. To counter rumors and misgivings about the vaccine, social mobilizers, religious leaders, and parents were included to increase understanding about immunizations.