The words epidemic and pandemic are referring to the spread of an infectious disease among a particular population. The difference between an epidemic and a pandemic is a dual one. Firstly, a pandemic is typically used to describe an indication of a much higher number of people affected than an epidemic, and, also, a pandemic refers to a much larger region than normal being affected. In the most severe case, a pandemic affects the global population.
An epidemic is generally defined by a disease, sickness or health-related issue that is apparent in more cases than would normally be expected. However, in the case of a pandemic, a lot more of the population is affected.
Hypothetically, let us assume that several people contract the same flu-like symptoms in a particular region. Then further assume that more cases show up around the area, but they remain localized in a few original cities or towns. Some cases may even turn up elsewhere in the country, but the sickness doesn't spread to anywhere else. In the locales where it has been reported, the infection rate remains higher than you would normally be expected. This is a typical example of an epidemic.
Take that same scenario but imagine that the incidence of infection has started growing exponentially so that more and more cases are cropping up locally. When the rate of infection grows very quickly it is likely, given propitious circumstances, that the epidemic will grow into something more serious. Now we may start seeing cases all across the nation and the rate of infection is moving beyond even that of an epidemic. In this hypothetical scenario most of the population in the nation has become affected by this illness. This is a pandemic.
However, even if the entire nation was affected but the rate of incidence doesn.t rise above that of an epidemic, it would still be regarded as an epidemic, even though the disease has spread nationwide.
On the other hand, if there was a small population in a remote region of Africa, for example, that has been nearly 100% affected by a health problem, because the incidence is so high, and the area comparatively widespread, affecting an entire population, this could be labelled a pandemic.
These subtle but significant differences can be confusing, but, generally an epidemic that has grown out of hand, due to the nature of the illness and various other factors, are considered to have turned into a pandemic.
A regionally localized disease or illness, if it involves more cases than a epidemic, can still be considered a pandemic; conversely, even though an illness might be widespread, if not enough of the population is affected to term it a pandemic, it will still be termed an epidemic. However, some may still label it a pandemic just because it is so far-reaching in a geographical sense.