When does a disease reach epidemic proportions? Usually, when a contagious disease gradually spreads among a very large population within a very brief period, for instance a fortnight or even less, it is called an epidemic. For instance, when meningococcal infections spread at a pace of over 15 cases in every 100,000 people for two straight weeks, it is said to have reached epidemic proportions.
Usually several factors are responsible for epidemic outbreak of contagious diseases. These may include a modification in the ecosystem of the host populace (for instance, increased stress or any augmentation in the vector species' density), a genetic modification in the pathogen pool or beginning of a rising pathogen to any host populace (by means of any movement of the disease-bearing microbe or the host). Normally, an epidemic takes place when the immunity of the host to an already established pathogen or a new and emerging pathogen reduces all of a sudden, much lower than the prevalent equilibrium and exceeding the brink of transmission.
The nature of epidemics may vary. It may sometimes be confined to one particular location. However, when an epidemic spreads to neighbouring as well as other countries, sometimes even other continents, and inflicts an extensive population, it may be called a pandemic. Before declaring a disease as an epidemic, it is necessary to have a proper understanding regarding the baseline incidence rate. For instance, epidemics for specific diseases like influenza are classified as attaining several defined enhancement in incidence, which is much higher that this criterion. A number of instances involving very rare diseases may also be defined as an epidemic. On the other hand, several cases of familiar diseases like common cold would not be classified as epidemics.
The use of the term "epidemic" possibly dates back to the Greek physician Hippocrates, usually referred to as the "Father of Greek Medicine". Hippocrates used this term no less than thrice in his concise book titled "Of the Epidemics". This book is basically a collection of descriptions of several medical case histories. In each case, Hippocrates used the term "epidemic" to depict the seasonal flare up of contagious diseases. However, he had little knowledge regarding the etiology (study of causes) of infections.
For over two millennia, people have used the term "epidemic" to denote any sudden outbreak of a disease or symptoms related to any disease in a population. In due course, the term was used to denote a single disease instead of a group of diseases or cluster of symptoms. However, it is interesting to note that the basic meaning of the word remained same for more than 2000 years. The main definition of "epidemic" in the Oxford English dictionary is widespread occurrence of any infectious disease in any community at any given time.
Despite being unclear, this definition of epidemic nicely portrays the common belief that epidemics are related to a surge in contagious ailment in any particular population. The Oxford Companion to Medicine's first edition offers a more specific description of the term epidemic, which has been defined as the occurrence of any disease for a certain period, wherein a considerably large number of people in any region or belonging to a community are inflicted by it compared to the number of people affected by the disease in any normal situation. It further states that the period as well as the severity of an epidemic is decided depending on the interaction of variables like infectivity and nature of the casual agent, the transmission mode of the disease, the extent of the already existing as well as the recently acquired immunity and others.
It is worth mentioning here that many modifications may take place in a contagious agent, which may also be responsible for triggering an epidemic. Such changes may include augmented virulence, modifications in the susceptibility of the host to the contagious agent and beginning of a new setting.
It is important to note that an epidemic disease does not need always be an infectious one. Several organizations, including the World Health Organization, have used the term to denote the West Nile fever as well as obesity epidemic, in addition to other ailments.
Many conditions are responsible for or control outbreaks of epidemics. Some of them include contagious food supplies like polluted drinking water and migration of particular animal populations like mosquitoes and rodents. These animals often serve as disease vectors. Moreover, it has been found that some diseases occur only in particular seasons.
These are numerous instances of this. For instance, whooping-cough strikes during spring, while measles may occur in epidemic form twice a year - once in winter and then in March. On the other hand, diseases like the common cold, influenza, and various upper respiratory tract infections like sore throat mainly occur during the winter. In addition to these, there is another distinct difference between epidemics and pandemics vis-à-vis the number of people suffering from epidemics as well as the deaths caused by successive epidemics. It has been found that the ruthlessness of consecutive epidemics rise as well as fall at intervals of about five to ten years.
As far as the spread of contagious disease among a community or population is concerned, both the terms epidemic and pandemic are somewhat same. However, epidemics and pandemics have two major differences. Usually, the term "pandemic" is used when much more people are affected by the outbreak of any disease compared to an epidemic. Moreover, when we use the term "pandemic", we actually mean that people in a much vast area are affected by a disease outbreak. In the worst possible instance, a pandemic may affect the entire population of the earth.
Generally, the words epidemic and pandemic also denote the rate at which an infection is spreading as well as the area it affects. They may also refer to both. When we use the term epidemic we mean that a particular disease or health condition is occurring in more cases compare to what is expected normally. On the other hand, the term pandemic refers to a much larger population being affected by a disease compared to the number of people inflicted by an epidemic. Usually, a pandemic occurs over a vast area, and is not restricted to any particular country or continent.
In order to imagine what the devastation an epidemic can cause, you may take a supposed case of numerous people are affected by common symptoms that are usually associated with flu in a particular region. Gradually, more similar cases appear all over the area, but the intensity as well as the spread of the disease is confined to few cities. Subsequently, a number of cases show up in other regions, but usually the disease does not spread to any other place. However, the infection rate remains very high compared to normal instances in the localities where the disease shows up. This can be seen as a typical instance of an epidemic.
Another more prevalent instance of epidemic would be an exponential growth in the infection with an increasing number of cases coming up every day. When the conditions are favourable, the infection rate increases more rapidly. In such situations, new cases may crop up in various other regions, while the infection rate would normally surpass the normally expected rate during an epidemic. In a number of situations, almost the entire population of a country, or maybe even the population in some other countries, may be affected by the disease outbreak. When this occurs, it is called a pandemic.
When a disease affects people all over a country, but its rate of incidence is not very high, it is still considered to be an epidemic. On the other hand, any disease that inflicts an extremely high fraction of a small population in a large region, for instance any remote region in Africa, it may be dubbed as a pandemic. Provided a disease is localized, but has inflicted more people than that in any epidemic, it may be described as a pandemic. On the other hand, an epidemic occurs when a disease inflicts people in a large area, but there not enough incidences. However, in the second instance, some people may still call it a pandemic simply because the disease affects people in a very large area.
In fact, this may seem to be confusing to many, especially the slight difference in how the terms pandemic and epidemic are used by people. However, in almost all cases, epidemics eventually develop into pandemics by spreading steadily at a rapid pace. In fact, the nature of these contagious diseases is such that they grow exponentially. Aside from the spread of diseases, epidemic is also used to denote rampant use of drugs. Sometimes, the term epidemic is also used metaphorically to denote harmless occurrences. However, the term pandemic is unlikely to be used in this way.
In simple words, when an epidemic becomes uncontrollable, it is termed as a pandemic. However, there are two subtle differences:
When the spread of an epidemic is not restricted to any small town or region, but covers a vast geographical region, it is usually known as a pandemic.
Sometimes, an epidemic may be confined to a small area, but it may affect a very large number of people that what is usually expected. In such a situation, the epidemic may be termed pandemic despite not spreading to a vast geographical region. For instance, the expected infection rate of a disease may be 15 percent, but when it affects about 40 percent of people in a region, it is called an epidemic. Similarly, when a larger number of people, say about 75 percent of the populace, is infected, it is basically a pandemic.