According to the World Health Organization (WHO), a pandemic can start when three conditions have been met:^[1]

• Emergence of a disease new to a population.
• Agents infect humans, causing serious illness.
• Agents spread easily and sustainably among humans.

A disease or condition is not a pandemic merely because it is widespread or kills many people; it must also be infectious. For instance, cancer is responsible for many deaths but is not considered a pandemic, because the disease is not infectious or contagious.