A pandemic is characterized by the global spread of a novel disease strain that has the potential to cause serious illness and elevated mortality in the human population. The ability of any pathogen to infect people and inflict illness or death is dependent on that pathogen’s virulence and transmissibility. For example, certain diseases such as influenza can cause serious illness or death, and are easily spread from person to person. Other diseases, such as Ebola virus, that cause very high mortality rates but are less readily transmissible.

However, not all people are equally at risk from pandemics. This is because pandemic emergence is more likely under certain conditions. For example, geographic regions where many people live that also have large populations of animals that serve as “mixing vessels”—which, depending on the pathogen, can include pigs, domestic poultry, or bats—are more likely to ignite pandemics. In addition, a person’s chance of developing serious illness after contracting a disease is dependent on that person’s age, sex, and general health. Access to supportive medical care, antivirals, antibiotics, and vaccines also plays an important role. Travel restrictions may help by slowing the spread of a pandemic, allowing the dissemination of medical supplies and vaccines before the first wave of illness arrives. In fact, if mitigation measures are promptly enacted, many deaths may be prevented.