Every day, millions of people around the world pack their bags and board airplanes — and they may be bringing contagious diseases with them. It wasn’t always this way, as people were once far less connected. Global travel has accelerated the spread of diseases, keeping public health officials on their toes.

SARS, Ebola and avian influenza are examples of communicable diseases that originated in one part of the world and appear to have spread to others via air travel. A 2016 study in The Lancet Infectious Diseases estimates that because of air travel, 2.6 billion people in Africa and the Asia-Pacific region — places with the right kind of mosquitoes and climate — may be at risk of being infected with the ZIka virus. Infected individuals are the carriers, and the airplanes are a common mode of transporting a disease to a new location far from the outbreak zone (the Americas, in the case of Zika). 

Aside from carrying disease, airplanes can also rapidly distribute lifesaving resources to disaster victims and deliver organs to transplant patients.