Final answer:
A pandemic is a widespread disease that affects many people at once, often across continents or worldwide. The correct answer is A. Pandemics are different from epidemics, which are more localized, and endemic diseases, which are consistently present in a population.
Step-by-step explanation:
The definition of a pandemic is a widespread disease that affects many people across multiple populations or continents, and can even be worldwide in scale. Thus, the correct answer to the question is: A. A widespread disease that affects many people at once. In contrast to a pandemic, an epidemic refers to a disease that occurs in an unusually high number of individuals in a population at the same time but is typically confined to a specific area. An endemic disease is one that is constantly present within a population, usually at low incidences.
Throughout history, there have been several pandemics such as the Black Death and the influenza pandemics of the 20th century. It is important to note that a disease can be caused by various agents, including viruses and bacteria, but not all viral or bacterial infections will reach pandemic status unless they spread significantly and affect a large proportion of the population.