Epidemics of infectious diseases arise due to spread of the disease across individuals. It spreads within a geographical region over a period of time. When they spread at the global level, it is referred to as a pandemic. An infectious disease originates at a particular point. However, it spreads across communities and geographical regions. The temporal and spatial patterns of spread of different infectious disease are unique. As a result, no two diseases, at any time or region will have the same trends. Also, infectious disease epidemics often result in widespread morbidity (condition of disease) and mortality (death). These depend upon the characteristics of agent or pathogen, host population and environment (Dicker et al. 2012). In this article, the common factors that influence the spatial and temporal patterns of disease epidemic are explored.
Why is knowledge about associated disease factors important?
A common outcome of every epidemic (infectious or non-infectious) is disease burden. Diseases outcomes within a population vary over time and space. This is because a wide range of factors contributes to the disease trends and distribution. These outcomes include health parameters like morbidity, mortality and economic parameters. Economic parameters include financial costs for individuals. Also, a country’s economic performance and government expenditure on social welfare programs are considered (WHO 2009). The government and public health organizations undertake prevention and control of epidemics. In order to provide timely and effective healthcare, they require up-to-date information regarding the disease and epidemic trends.
Epidemics do not appear randomly in a population, but due to risk factors distributed heterogeneously. These factors place several individuals at health higher risk than others (Dicker et al. 2012). However, factors that contribute to the spread of disease can be different from those that lead to outbreaks. This is because disease outbreaks occur in a single community but spread to multiple communities and regions. Here, ‘disease outbreak’ refers to the starting point of disease. Therefore factors specific to multiple communities or regions will be involved in epidemics over single community characteristics. Such factors include inherent characteristics of the pathogen and host. Moreover, environmental factors are also involved. This is shown in table 1 below (Thomas & Weber 2001).
|Pathogen or agent||Host||Environment|
|Intrinsic||Extrinsic or Behavior||Physical||Social|
|Pathogenicity||Age||Diet||Urban or rural||Travel|
|Infectivity||Gender||Habits (Drugs, alcohol, smoking)||Tropical or temperate||Crowding|
|Infective Dose||Race||Sexual activities||Climate||Medical availability|
|Immunogenecity||Genetic Makeup||Occupation||Remoteness||Sexual networks|
|Evasiveness||Physiology||Recreational activities||Vector Presence||Public health resources|
|Environmental Stability||Immune responsiveness||Animal exposure||Education|
|Vector Necessity||Immunosuppresive medications or chemotherapy|
Table 1: Characteristics of host, pathogens and environment contributing to infectious disease epidemics
Role of interactions in disease transmission
These factors are all connected to each other through interactions. This can transpire either human-human, agent-agent, agent-human, agent-vector, vector-human or others. It is shown in the figure below. Unique interactions among these are human-human which help in spreading the epidemic. Agent-agent is where two variants or subspecies of a pathogen can combine to form a new mutant. This is also problematic. A prime example of this is the Influenza virus which continuously evolves and mutates into a new variant. Each variant has varying infectivity and mortality rates (Taubenberger & Kash 2010).
Outbreaks are the starting point of epidemics of infectious diseases. They are caused by different factors. There are three major factors that lead to an outbreak in a community (WHO, 2009). This is shown in the figure below.
Therefore, interaction patterns and environmental characteristics play a significant role in the outbreak of diseases. Knowledge of these factors is advantageous in the analysis and prediction of epidemiological data (Ughade 2013). Moreover, the government can plan control and prevention strategies keeping in mind these factors.
Studying the factors that contribute directly or indirectly to a disease outbreak. It can also help determine the risks of population or population sub-groups. Moreover, it can assist in the understanding extent of contribution. Planning and prediction of success of control or prevention strategies can be done. Such models can help study interaction dynamics existing within a population. Host and agent behaviour and also environmental characteristics can be assessed (Siettos & Russo 2013). Mainly, agent or pathogen, host and environmental features contribute to disease outbreaks and spread.
Features of agents or pathogens responsible for disease outbreaks
The common pathogens causing infectious diseases are bacteria, virus, protozoa, fungi, helminths and prions. Each of them belongs to a different class of organisms. Pathogens or agents possess intrinsic characteristics that dictate their pathogenicity, host range and mode of transmission. ‘Pathogenicity’ here refers to the proportion of infections leading to a diseased state. These include tolerance to environmental factors, mode of reproduction, size, defence against host and susceptibility to antibiotics (Thomas & Weber 2001). Extrinsic characteristics of pathogens based on host-agent interaction include:
- Infectivity (the ability of the pathogen to invade and multiply in the host),
- Infective dose (no. of organisms required to establish infection) and
- Route of transmission.
These factors contribute to pathogenicity. In emerging infectious diseases the ability of the pathogen is very important. These abilities include adaptation and evolution in hosts or vectors through mutation and human contact (Leventhal et al. 2015).
Characteristics of hosts responsible for disease outbreaks
One of the prominent factors driving infectious disease epidemiology is host resistance or susceptibility. Resistance or susceptibility naturally depends upon the state of the immune system. This includes both specific and non-specific immunity. Abnormalities can arise due to innate condition (sickle cell anaemia) or acquired (malnutrition or infection) or immune-suppression (Thomas & Weber 2001). Besides this, a demographic profile like age, gender, race and genetic makeup of a population affect infection. Extrinsic factors of hosts include behaviours like diet and history of infection. Also, network-based interaction with other humans, travel, drugs or alcohol or smoking and sexual activities are responsible (Thomas & Weber 2001).
Environmental factors leading to disease outbreaks
The major environmental factors leading to disease outbreak are climate and extreme natural events. Anthropogenic factors like sanitation, utilities also play a role (WHO 2017). Environmental factors include all factors related to agent-host interaction, mainly physical, biological and socioeconomic (Thomas & Weber 2001). These factors influence the behaviour and ranges for agent, vectors and hosts. Agent reservoirs are also affected. Physical factors include climate (humidity, temperature, precipitation trends) and topography of the area. Biological factors include plants and animals that humans frequently interact with. Finally, socioeconomic factors of environment influence human behaviour and interaction and hence diseases transmission. Common factors include population density, housing, occupation, sanitation, education, culture and availability of health services (Thomas & Weber 2001).
Challenges posed by interaction behaviours
One can determine causative factors through observation and laboratory analysis. However, the complex interactions existing between all organisms and factors are hard to interpret. When multiple factors simultaneously lead to disease transmission, the route of transmission is hard to determine. Knowledge about these interaction patterns is important in predicting the success of control strategies. Governments should spread knowledge at the individual, societal and environmental levels. Moreover, prediction of future events or early warning systems also requires comprehensive knowledge about associated factors and the risks they pose. Therefore, risk prediction based on associated or causative factors is needed along with interaction studies or modelling in disease epidemiology research.
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- Leventhal, G.E. et al., 2015. Evolution and emergence of infectious diseases in theoretical and real-world networks. Nature Communications, 6, p.6101.
- Siettos, C.I. & Russo, L., 2013. Mathematical modeling of infectious disease dynamics. Virulence, 4(4), pp.295–306.
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- Thomas, J.C. & Weber, D.J., 2001. Epidemiologic Methods for the Study of Infectious Diseases, Oxford University Press.
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- WHO, 2017. Environmental Health in Emergencies: Disease outbreaks. WHO. Available at: http://www.who.int/environmental_health_emergencies/disease_outbreaks/en/.
- WHO, 2009. WHO Guide to Identifying the Economic Consequences of Disease and Injury., Geneva, Swizerland.