When referring to the spread of diseases, we so often hear the terms outbreaks, epidemics, and pandemics, but what do these terms actually mean and what are the differences between them? And who has the authority to ‘upgrade’ the threat level of a disease from an epidemic to a pandemic?

Dr Benjamin Bailly, Research Scientist at the Institute for Glycomics, answers all these questions and more, providing a very interesting overview on epidemiology – the study and spread of diseases.

What is epidemiology?

Epidemiology is the study of diseases or other health conditions and their impact on populations, from understanding the way they spread to who they affect and how.

What are the aims of an epidemiologist?

For ease of understanding, an epidemiologist can be described as a ‘disease detective’. When disease outbreaks occur, the disease detectives are sent to the ‘scene of the crime’, to investigate things like:

  • What caused the disease outbreak
  • Who is sick and who is at risk of getting sick
  • What are the common symptoms
  • How is the disease spread
  • How can we control the disease or stop its spread

Do epidemiologists only focus on infectious diseases?

Epidemiologists do not only focus on infectious diseases. They also focus on other health issues such as cancer, diabetes and other disorders, as well as any health-related issues that may positively or negatively impact us.

Why is epidemiology referred to as an interdisciplinary field?

Because epidemiology studies so many aspects of a health-related issue, it relies on the expertise of individuals from various disciplines. For instance, it may rely on medical doctors and diagnostic scientists in the field, as well as molecular and evolution biologists, to understand how pathogens evolve, or mathematical modelling experts to predict how the disease will spread.

What is meant by the prevalence and incidence of disease?

The prevalence of disease is, at a given time or during a given period, the proportion of individuals affected by the said disease. It includes both new and existing cases.

The incidence of disease only refers to new cases. It is the rate at which new cases emerge during a given period, or the proportion of new cases.

Understanding the difference between disease morbidity and mortality

Morbidity is the state of suffering from an illness, usually manifesting with symptoms.

In contrast, mortality refers to disease-related death.

Understanding the differences between outbreaks, epidemics, pandemics and endemics

  • Outbreaks

An outbreak is the sudden occurrence of multiple cases of a disease that is beyond usual levels. Outbreaks are usually localised, such as in a school or a city, and may involve only a handful of cases.

Outbreaks of hand, foot and mouth disease, for instance, may cause day care centres to close temporarily as the disease is contagious and spreads quickly. Another example is the recurring outbreak of Ebola in West Africa, usually contained within localities. As it may linger for long periods of time or affect multiple countries, it can also be referred to as an epidemic.

  • Epidemics

An epidemic will spread to affect a larger area and a greater proportion of the population than an outbreak, such as one or more cities or countries.

Epidemics typically apply to infectious diseases, but they can also apply to non-communicable diseases such as cancer, cardiovascular diseases or obesity.

For example, the influenza virus causes seasonal epidemics of influenza throughout the world, that may affect multiple countries. On the other hand, a nuclear catastrophe may be the source of an epidemic of cancer in the affected population.

Epidemics are of particular concern in poor and developing countries because of the restricted access of populations to disease prevention measures and health care.

When an epidemic cannot be contained, it will lead to a rapid increase both in the number of cases and in the size of the affected area. If the disease becomes global and spreads to multiple countries and continents, it may be called a pandemic.

  • Pandemics

The word pandemic originates from the Greek “pan”, meaning “all”, and “demos”, meaning “people”.

Pandemics differ to epidemics by their scale. They usually affect a great proportion of the global population, over multiple continents.

Like epidemics, pandemics technically do not only apply to infectious diseases. In most cases though, a pandemic will refer to an infectious disease.

A very obvious example is the current worldwide COVID-19 pandemic that is affecting almost every country on the planet. Other examples are the 1918 influenza pandemic that killed 20-50 million people worldwide, or the Black Death caused by the bacterium Yersinia pestis that caused 75 to 200 million deaths during the 14th century.

  • Endemics

An endemic disease refers to a disease that can be found in specific areas or populations, with a stable incidence. In other words, in an endemic the number of cases remain somewhat stable.

A notorious example of an endemic is chicken pox in the United States, which affects children every year at a predictable rate without growing out of control. Another example is malaria in some countries where the number of cases remain stable. In other areas though, the incidence of malaria continues to increase and causes local epidemics.

So, what is the difference between an endemic and epidemic?

An epidemic leads to an unusual increase in numbers of cases of a disease, while in endemics these numbers remain stable and at baseline levels.

Who declares a global pandemic?

The World Health Organization (WHO) has the authority to declare an epidemic a pandemic if it spreads over several countries or continents and affects a large proportion of the population.

When a pandemic is declared, countries and public health agencies will have increased capacities to manage the pandemic. There are no particular events triggered by the declaration of a pandemic, mostly a global realisation of the seriousness of the situation and a call for action to manage the crisis as best as possible.

What are some of the potential impacts of global pandemics?

A global pandemic may disrupt the economy, as seen in the case of COVID-19 and the associated lockdown measures, lead to an increase in overall deaths, or have negative social and psychological impacts.

Why has the risk of the development of new pandemics increased drastically in the 21st century?

The risk of the development of new pandemics has increased in the 21st century for various reasons. One of them is that as the global population continues to increase, so does the frequency of contact between individuals. These events may contribute to disease transmission.

Another issue is the disruption of the environment due to globalisation, that may lead to deforestation for instance. Such events force animal species to move out of their natural environment and may contribute to the transmission of pathogens from animals to humans. This transmission is called zoonosis and has been responsible for a number of epidemics throughout the world such as bird flu, AIDS, Ebola fever, SARS, MERS and COVID-19.

Another contributing factor is the ease and increasing modes of international travel available in this modern age. With the ease and frequency of global travel, an infectious disease can spread rapidly around the world and within a short period of time.

What does the new term infodemics refer to and why is this a growing concern in the digital age?

Infodemics refers to the very rapid spread of information through digital platforms. This information may be accurate as well as inaccurate, which is why it is of particular concern in our modern digital age.

As we have seen with the current COVID-19 pandemic, a lot of misleading and false information was spread through social media, which has sometimes negatively impacted the public’s behaviour towards the disease.

Infodemics also give rise to an accelerated spread of conspiracy theories that undermine the efforts of experts in effectively managing the pandemic.


Dr Benjamin Bailly is a virologist specialising in the discovery of novel antivirals. He utilises multidisciplinary approaches to discover inhibitors and understand their mode of action. Dr Bailly currently works on sugar-based strategies for the inhibition of enteroviruses causing hand, foot and mouth disease (HFMD), as well as on the discovery of new-generation influenza virus and human respiratory syncytial virus inhibitors. In the HFMD space, he extends his work to the characterisation of the interactions between enterovirus and host sugars and their role in cell tropism. Dr Bailly recently joined a task force to identify novel treatments and vaccines against COVID-19 (SARS-CoV-2 infection).