Causes of Epidemics

Cong T. Nguyen


Emerging and re-emerging epidemics have been posing an extreme threat to global health security. An epidemic is defined in the Oxford Advanced Learner’s Dictionary as “a large number of cases of a particular disease happening at the same time in a particular community” (“Epidemic”, n.d., para. 1). In reality, at different times and in different places, outbreaks of epidemics would cause diseases, disability, and deaths as well as destabilize global economy and population. The Modern Plague, for instance, occurred in the 1860s and caused over 12 million deaths in China, India and Hong Kong (“Deadly diseases,” 2014). Recently, the World Health Organization has revealed “closely to 27,000 suspected or confirmed cases and over 11,000 Ebola-related deaths have been recorded” (World Economic Forum [WEF], 2005, p. 4). Epidemic diseases have been becoming an extreme warning on the global scale and the situation may even become worse in the future. This essay characterizes some major factors contributing to epidemic outbreaks and spread as well as devastating consequences of epidemic diseases.

The Epidemiologic Triangle or triad is a model of three main factors contributing to epidemic outbreaks and spread, including agent, host and environment. “The agent is whatever causes the disease. The host is where the agent lives. And the environment is all the rest of the agents’ and hosts’ surroundings” (Baylis, n.d.). In some cases, a third party called the vector is required for the agent to get access to the host. (The Pennsylvania State University [PSU], n.d.; Rockett, n.d.). According to Miller (2002), the model can be used for non-infectious diseases of which the agent could be “unhealthy behaviours, unsafe practices, or unintended exposures to hazardous substances” (as cited in Wilson, 2013, para. 1).

Agent – Ecological factor

The agent is one of three elements in the Epidemiologic Triangle. An agent causing infectious diseases could be a bacterium, a virus, a parasite or other micro-organism. For example, a bacterium named Yersinia pestis results in plague disease which is notorious for killing millions of European people during the Middle Ages (Centers for Disease Control and Prevention [CDCP], 2015). As time went by, “the concept of agent has been broadened to include chemical and physical causes of disease or injury”, including chemical contaminants and physical forces (CDCP, 2012, para. 5).

Host – Social factor

The host or social factor in the triad refers to a susceptible human or animal bearing the agent. The host has numerous typical characteristics known as host factors which can affect “an individual’s exposure, susceptibility, or response to a causative agent” (CDCP, 2012, para. 6). Considering the host and the environment as key factors, A Lee (2003) wrote:

The susceptibility of the host depends on its ability to fight off the infection, which can be a disease specific defence mechanism such as vaccine, or non-specific defence mechanism. The ability of non-defence mechanism to fight off infectious disease will depend on the host’s general health status, nutritional status, age, coexisting chronic illness, etc. If you have a population that is healthy, fit, and well nourished, the chance of infection would be low.


The environment includes external factors affecting the agent and the possibility of exposure. Generally, these factors are categorized as the social, physical or biological environments. The social environment includes factors like education, unemployment, economy, transportation and availability of health care service; the physical environment may include factors like climate, land and contamination; biological environmental factors may be vectors, humans and plants functioning as reservoirs of infection (Kebede, 2004). The favorable or unfavorable environment is essential to the survival of the agent, its ability to infect the host and its transmission (Lee, 2003). For example, eggs of hookworms in excrement may hatch under favorable environment involving moisture, temperature and soil type, which makes a contribution to the cycle of hookworm disease transmission (Cliff, Haggett & Smallman, 2004).


Throughout history, outbreaks of epidemic diseases resulted in enormous declines in the number of the inhabitants in or around the infected areas. Smallpox in 430 BC, for instance, killed over 30,000 people in Athens, Greece, which was relevant to at least 20% of the city’s population. Striking again in 1519, the Smallpox epidemic caused between 5 to 8 million deaths in the following two years. The Plague of Justinian, which began in 541 AD and lasted nearly 200 years, was estimated to claim 50 million lives in the Middle East, Asia and the Mediterranean basin. The human immunodeficiency virus or HIV, the cause of acquired immune deficiency syndrome (AIDS), has killed more than 25 million people since the first cases were reported (“Deadly diseases,” 2004). Facing extreme effects of epidemics in various types, human life has constantly been put at a deadly risk.


Infectious diseases have negative influences on national and global economy in terms of medical costs and foregone income. One of the most outstanding impacts of epidemics on the economy is its increasing burden of medical costs including private and non-private medical costs. The rise of the costs is attributed to the requirement of keeping disinfected environments, taking prevention measures, and carrying out elementary research (Knobler et al., 2004). For instance, Frangoul (2014) noted that £424 million ($708 million) has been spent on flu drug Tamiflu to prepare for a flu outbreak by the United Kingdom (UK) government. The medical costs are even far more expensive to tackle a pandemic which is an epidemic spreading and having impacts on global scale. Frangoul also stated that “the cost to the global economy of SARS is estimated to have been $54 billion, according to the World Bank, while the organization estimates that a ‘severe flu pandemic’ could cost over $3 trillion, nearly five percent of global GDP” (2014, para. 7).

Economic consequences of epidemics also include foregone income due to either disease-related morbidity or mortality. Forgone income is usually measured by “the value of workdays lost due to the illness”; however, in circumstances of mortality, it is assessed by “the capitalized value of future lifetime earnings lost to the diseaserelated death, based on projected incomes for different age groups and age-specific survival rates” (Knobler, 2004, p. 94). This cost can be significant to some epidemics. For example, the World Health Organization (2002) reported that an estimated 3.1 million people died of HIV/AIDS-related causes worldwide in 2002.


Epidemics stem from three main factors including an agent, host and environment. Therefore, an epidemic can be stopped not only by medical treatment but also by changes in the characteristics of the agent and host factors. These changes can be done by passing laws, changing living conditions, educating the community and other communal, legitimate and governmental actions (Baylis, n.d.). However, agents causing diseases continuously change to survive, and the battle against epidemics is even becoming more difficult; for example, scientists have recently found bacteria that are completely resistant to treatment, also known as the antibiotic apocalypse (Gallagher, 2015).

Epidemics have instantly been of great concern to human because of its causing incalculable death and economic losses. As the situation keeps continuous and may get worse in the future, human beings are trying to win the battle. Our insight into the future is possibly getting out of the shadow of epidemics or facing endless increasing numbers of patients.


Baylis, E. A. (n.d.). Week 1 video 2: Why epidemics happen [Video file]. Retrieved from

Centers for Disease Control and Prevention (2015). Plague.

Centers for Disease Control and


Retrieved from

Centers for Disease Control and Prevention (2012, May 18). Principles of Epidemiology in Public Health Practice, Third Edition: An Introduction to Applied Epidemiology and


Centers for Disease Control and Prevention

. Retrieved from

Deadly diseases: Epidemics throughout history. (2014, October).

Cable News Network

. Retrieved from

Epidemic. (n.d.). In

Oxford Advanced Learner’s Dictionary

(9th ed.). Retrieved from

Frangoul, A. (2014, February 5). Counting the costs of a global epidemic.


. Retrieved from

Gallagher, J. (2015, November 19). Antibiotic resistance: World on cusp of ‘post-antibiotic era’.

BBC News

. Retrieved from

Kebede, Y. (2004).


. Retrieved from

Knobler, S., Mahmoud, A., Lemon, S., Mack, A., Sivitz, L. & Oberholtzer, K. (2004).


from SARS: Preparing for the Next Disease Outbreak

. Washington, DC: The National Academy Press.

Lee (2003). Will the SARS epidemic recur?.

Journal of Epidemiology & Community Health.


(10). Retrieved from

Rockett, I. R. H. (2009). Descriptive Epidemiology for Public Health Professionals

. Sudanese

Journal of Public Health. 4

(3), 354

The Pennsylvania State University. (n.d.). Epidemiologic Triad.


. Retrieved from

Wilson, S. (2013, October 24). Epidemiology: An introduction.

Open Learn

. Retrieved from

World Economic Forum (2015).

Managing the Risk and Impact of Future Epidemics: Options

for Public-Private Cooperation

. Geneva, Switzerland: Author.