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Communicable (infectious) diseases are diseases that are contagious. That is, they are infections or diseases that can be transferred or passed on from one person (sick person) to another individual (susceptible host). Communicable or infectious diseases are diseases that are naturally transferred from one person to another by a disease-causing agent or pathogen 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 amongst people in the developing world (e.g., Africa, South America, Asia), 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 or pathogen from one host to another. This ensures that the microbe goes on to affect other hosts and thus, continues their life cycle in the process. 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 coevolved 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. Communicable diseases can either be zoonotic diseases or nosocomial diseases that are transmissible from animals and hospital environments respectively in a defined human population.

Table 1. Emerging and Re-emerging Bacterial Diseases

DiseaseYear of outbreakCountryTypeClinical features
Legionnaires’ disease1976Philadelphia, USAEmergingFever, headache, and malaise  
Haemolytic uraemic syndrome1982North America, Europe & Japan  EmergingEnteritis, stomach upset, inflammation of the intestines 
Lyme disease1975Connecticut in USAEmergingMalaise, fever, , fatigue, chills, neurological abnormalities, heart inflammation, arthritis and multiple sclerosis  
Cholera1992India and AfricaEmergingProfuse stooling (watery), loss of weight, loss appetite and rice-water stool  
Diphtheria1996USA & RussiaRe-emergingInflammation of the tonsils, pharynx and nose  
Meningococcal meningitis1990-2004Sub-Saharan regions of AfricaRe-emergingFever-like symptoms and inflammation of the meninges of the brain  
Extensively drug resistant tuberculosis (XDR TB)  1990 – till dateOne-third of the world’s populationRe-emergingRespiratory disease extremely resistant to available TB drugs
Campylobacteriosis1975United KingdomRe-emerging  Enteritis and diarrhea
Listeriosis1990-2000USA and EuropeEmergingEnteritis, meningoencephalitis, abortion and septicaemia  
Melioidosis1990Thailand, Laos, USA Australia, MalaysiaEmergingZoonotic disease of man and animals
  Anthrax  1990-2000  Tropical and subtropical countries  Re-emerging  Septicaemia and sudden death with the exudation of tarry blood from the body orifices of the cadavers and the failure of the blood to clot. Absence of rigormortis, and the presence of splenomegaly.  
Brucellosis1887Most countries of the world  Re-emergingFever and abortion in animals
Leptospirosis2000EuropeRe-emerging  Enteritis and diarrhea
Salmonellosis1959United Kingdom, USA and Europe  Re-emergingEnteritis and gastrointestinal upset
PlagueFirst occurred in the 6th century  South America, Mexico and USARe-emergingSepticaemia, pneumonia and lymphadenitis
Tularemia1910China, Japan, USA, France, Norway, Sweden and Canada  Re-emergingInflammation and swelling of some vital organs such as the eye’s conjunctiva
MRSA infection1981Africa, UK, Europe, Asia and USA  Re-emergingBacteraemia, septicaemia, and impetigo  
Clostridium difficile Associated Disease (CDAD)  1989 and 1992USARe-emergingEnteritis, watery diarrhea and severe abdominal pain; loss of appetite 
*COVID-192019WorldwideEmergingFever, cough, sneezing, difficulty in breathing, lose of smell, joint/body pain, pneumonia etc
*SARS-CoV-2= severe acute respiratory syndrome coronavirus strain/type 2
*COVID-19 = coronavirus disease 2019

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. Typical example of an emerging disease is the novel coronavirus disease 2019 (COVID-19) which is caused by SARS-CoV-2 (severe acute respiratory syndrome coronavirus strain/type 2). Emerging diseases are diseases that have the potential to also increase in their prevalence even in the near future, and across the world. 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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Nelson K.E and Williams C (2013). Infectious Disease Epidemiology: Theory and Practice. Third edition. Jones and Bartleh Learning. 

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Rothman K.J, Greenland S and Lash T.L (2011). Modern Epidemiology. Third edition. Lippincott Williams and Wilkins, Philadelphia, PA, USA.

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