The Sequence of Communicable Diseases

Communicable (infectious) diseases are diseases that are contagious. That is, they are infections that can be transferred or passed on from one person (sick person) to another individual (susceptible host). They are diseases that are naturally transferred from one person to another by a disease-causing agent that passes either directly or indirectly from the sick individual to a healthy person. Such diseases are transmissible, and can be transferred to a susceptible host through direct body contact with an infected person, contact with infected fomites or vectors and through contact with the body fluids or droplets of sick individuals. Infectious diseases are usually the most common cause of morbidity and mortality among people in the developing world (e.g. Africa), and this is attributable to several reasons including poor public health management, poor sanitation, lack of access to potable drinking water, overcrowded living conditions, poor sewage and refuse treatment, malnutrition, and lack of resources to put these things in place. Communicable diseases can be contained and eradicated in a particular population, community or nation when the fundamental living standards of the general population (in terms of improvement in water supply, waste and sewage management, improved nutrition and food supply) are improved upon.

The survivability of a pathogen is largely dependent on the successful transmission of the disease-causing agent from one host to another. This ensures that the microbe goes on to affect other hosts and thus, continues their life cycle. This is the case with most disease-causing agents that are implicated in a variety of infectious diseases. But if the pathogen has no other host to infect (especially in cases where host infectivity by a given pathogen results in the death of the host), then the death of the infected host will automatically result to the extinction of the disease-causing agent. Thus most pathogens co-evolved with their host, a scenario in which the development of the host affects that of the pathogen and vice versa. This enables the disease-causing agent to be passed on from one host to another, making a variety of pathogens to rely on a host-to-host transmission of the disease they cause within a defined human/animal population. Some microbial diseases are emerging in nature while others are re-emerging (Table 1). Emerging diseases are new infectious diseases caused by novel pathogenic microorganisms that are still evolving within a particular community or population while re-emerging diseases are old infectious diseases that are known but have surfaced again after a significant decline in their incidence in the past.

 Table 1: Emerging and Re-emerging Bacterial Diseases

Emerging diseases are generally the first outbreak of an infectious disease caused by unknown pathogenic microorganisms; and they also include infectious diseases that are known but whose incidence in a defined human population have significantly increased in frequency in the past twenty years. They are diseases that have the potential to also increase in their prevalence even in the near future. Emerging and re-emerging diseases include those infectious diseases caused by pathogenic virus, bacteria, fungi and protozoa. And significant number of these diseases is zoonotic in nature i.e. they can be naturally transmitted between humans and animals.       

Communicable diseases are usually transmitted to humans through disease-causing agents in any of the following two (2) ways:

  • Directly: from one infected human being or animal to another uninfected individual. This usually occurs following body contact of a susceptible host with an infected human or animal.
  • Indirectly: through vectors (e.g. rodents and insects), fomites, vehicles (e.g. food, water, milk, surgical tools), and through the air or dust particles.

A disease-causing microorganism must be able to reproduce and disseminate successfully in a particular human/animal population in order to maintain relevance in the infection and transmission of its disease from one host to another.


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