Final answer:
A pandemic is an epidemic that spreads on a worldwide scale. It occurs when a disease affects a very high proportion of the global population, as seen with HIV/AIDS. Understanding disease prevalence and distribution involves calculating averages and rates using age-standardized data.
Step-by-step explanation:
An epidemic is a widespread occurrence of a disease in a specific community or region at a particular time. A pandemic is a type of epidemic that is even more expansive – one that spreads across multiple populations or continents, or even worldwide. When scientists and researchers study infectious diseases and find that a high percentage of a population has been diagnosed with a disease, as noted with countries having rates ranging from 11.4 percent to 74.6 percent, this can be indicative of a pandemic if it occurs on a global scale. For example, the HIV/AIDS crisis is considered a pandemic because it has spread extensively around the world, affecting a significant portion of the population across various continents.
When calculating the average percentage of individuals affected by a disease in various countries, this would involve measures of the center of the data, often expressed as a rate. This data, such as the prevalence of a disease affecting 17.3 percent of people ages 15 to 49 in South Africa, helps to understand the distribution and impact of the disease on different populations. It is especially important when it's age-standardized to allow for accurate comparisons between countries and over time, as is done in the Global Burden of Disease (GBD) studies.