What is an epidemic?

'Epidemic' is a frightening word, but also a confusing one. In its strictest sense, it refers only to outbreaks of infectious disease, but the word is increasingly used in a much wider way in conversations about health issues such as obesity, type 2 diabetes, heart disease, depression, allergy and cancer, to name a few.

In an even wider sense, people use the word 'epidemic' to describe social trends that may have little or nothing to do with disease, such as cybercrime or poverty. You can even hear people talk about 'the selfie epidemic' or 'an epidemic of bike thefts'.

A common theme unites these different uses of the word 'epidemic'. It has surprisingly little to do with numbers of people affected; instead, it describes the unexpected. An epidemic is defined when there are more cases of a certain disease than expected.

Thus, an epidemic is an event that catches us by surprise, and produces an emotional response - fear, anger, compassion, religious devotion.

Infectious epidemics

In its strongest and most traditional form, an epidemic is an infectious disease spreading through a population. Infectious epidemics have been regular and significant events in human history - smallpox, plague, cholera, influenza and others have been the scourge of humankind for thousands of years.

Pathogens (disease-causing microorganisms such as viruses and bacteria) are humanity's constant companions, but most of the time they do not cause epidemics. A pathogen can circulate throughout a population for many generations, during which the pathogen and humans' immunity gradually develop an uneasy truce. In this situation, the disease is said to be endemic to the population - people will sometimes fall ill, but at a relatively stable rate.

How do infectious epidemics happen?

An epidemic of infectious disease occurs in one (or more) of the following scenarios:

1) An existing pathogen evolves into a new, more infectious strain

Pathogens change and evolve all the time. Once in a while, a change can increase a pathogen's capacity to cause harm, or its ability to spread easily from person to person.

For example, influenza is a special case of infectious disease, in that it causes regular epidemic outbreaks every year. This is due to the influenza virus's unique ability to change its properties. New strains of the virus are constantly being created and spread.

When we are infected with this new strain, it is unfamiliar to our immune system, so no immune memory exists and we get sick.

For more information, see our influenza page.

2) An existing pathogen arrives at a new, unprotected population

Travel and trade expose populations to new diseases. When a pathogen is introduced into a population that has never been exposed to it, the new hosts' immunity is unprepared for tackling it.

For example, smallpox, measles and cholera epidemics significantly affected indigenous American Indian and indigenous Australian populations who first came in contact with Europeans. Meanwhile, these Europeans brought syphilis from the New World back to their homeland.

3) A pathogen of animals 'makes the jump' between species to humans.

In rare instances, a pathogen that normally circulates in an animal species evolves to infect humans.

It is important to understand that a pathogen can often infect from an animal to a human, but cannot then move from one human to another. For a pathogen to be able to move from person to person, it needs to develop a new set of adaptations, and that is a rare event.

Such examples include:

• Rabies, which moves between mammals (particularly dogs and other canines). An infected animal can transmit the rabies virus to a human. However, the virus cannot pass from person to person, and;
• Avian flu (or 'bird flu'), a virulent type of influenza that caused hundreds of deaths since its appearance in the early 21^st century. At first, there was much concern that the virus could cause a widespread epidemic if it evolved to pass from person to person, but so far this has not happened. Nearly all people who contract avian flu are infected by birds (usually poultry) and do not pass the virus further.

Person-to-person infection does not necessarily mean that the pathogen moves directly between people. In other words, infectious epidemics are not necessarily contagious. Some diseases, such as malaria or West Nile virus, move from person to person through intermediates such as mosquitoes, which are common disease transmitters between humans.

4) A drop in a population's health status

In this case, a population's overall health suffers. These situations often occur in times of widespread hunger, war or natural disaster. A less healthy population is more susceptible to infectious disease.

War and natural disaster can also worsen a population's hygiene status. A common scenario sees a breakdown in sanitation, leading to the water supply being contaminated with sewage. Pathogens can then spread quickly throughout the population.

Epidemics, outbreaks, plagues, pandemics - what's the difference?

Outbreak

An outbreak is the initial event - a sudden rise in the number of cases of a health condition in a population. For example, the number of people arriving in a hospital's emergency rooms with intestinal complaints suddenly increases. Investigations may trace this back to spoiled meat from a single source (such as a restaurant or meat supplier) that all the ill people had eaten, or it can find that the cause is a virus that has begun circulating in the city.

An initial outbreak may be an isolated incident (food poisoning will not spread further once the spoiled food is taken care of), or it may progress and become an epidemic.

Epidemic

After the initial outbreak, an epidemic is declared when the disease spreads quickly to affect more people. Epidemics can be large or small events, affecting whole countries or a small community - a village or a school. An epidemic can run its course within a few weeks, or last for many decades.

Pandemic

A pandemic is an epidemic that is not confined to a single area, but spreads far and wide, crossing national borders and continents. The prospect of pandemics has become an increasing worry in recent decades, as more people travel farther and faster.

Plague

The word 'plague' has been used throughout history to describe both epidemics of any infectious disease, as well as any other widespread issue affecting a population, such as crop-destroying insects. Today the word 'plague' in medicine is reserved for a specific disease caused by Yersinia pestis bacteria.

Response to infectious epidemics

In the event of an epidemic, health authorities will take steps to:

• Contain it - prevent the epidemic from spreading further;
• Treat those infected;
• Find the sources of the epidemic and treat them if possible;
• Provide information and prevent unfounded speculation and panic, and;
• Collect and publish data about the epidemic, for use elsewhere or in the future.

Efforts at containing the disease can include physical measures (quarantining the ill, closing down roads and borders), but a large part of modern epidemic response is centered on educating and informing citizens - telling people what to do, what to avoid, how to recognize signs of illness, where to get help, and so on.

The biological realities of epidemics mean that it is not easy to predict how they will play out. With every epidemic that comes along, governments and health authorities run the risk of either underreacting to an epidemic, thus failing to contain it in time and allowing it to spread, or overreacting to it, costing valuable resources and increasing public alarm.

Though science and society have made great strides in reducing death and illness from infectious epidemics, they are ultimately a fact of life on earth. New variants of pathogens will continue to appear.

Non-infectious epidemics

Up until quite recently, infectious diseases were the main cause of death in humans. Improvements in medicine over the past century and a half - improved hygiene and sanitation measures, vaccination and the development of antibiotics - have meant that the burden of infectious disease is much lower today.

Although infectious diseases still take a dreadful toll, especially in developing countries, the health threats facing humanity have shifted to non-infectious causes. The incidence of heart disease, type 2 diabetes, obesity, cancer and other non-infectious conditions is now very large, so many find it useful to discuss these trends in terms of epidemics.

Of course, there is no single, identifiable agent that is responsible for diabetes or heart disease as there is for influenza or AIDS. The causes of these modern epidemics are diverse and include biological, societal, cultural and psychological factors. However, thinking about these diseases in terms of epidemiology can help us as a society to shape our response to them and find ways to reduce their incidence.

'Social' epidemics

The use of the term 'epidemic' to refer to human behaviors that are not related to disease is largely figurative; when we read of, say, a 'carjacking epidemic' in our city, we do not assume that a carjacking virus is spreading through the population, causing people to carjack. Instead, we understand that there has been an alarming rise in carjacking incidents lately and that something must be done about it.