Introduction to communicable diseases

Introduction to communicable diseases

Introduction to communicable diseases

Communicable diseases, also known as infectious diseases or transmissible diseases are diseases  that spreads from one person or animal to another or from a surface to a person

Communicable diseases occur at all age groups outmost serious in childhood due to intensive exposure and poorly developed immunity. These diseases are to a great extent preventable

In countries where the disease have been largely prevented, other conditions like accidents, and degenerative and malignant diseases that occur at an old age have become the commonest, the process known as epidemiological transmission

Tropical countries, Uganda, inclusive have continued to struggle with poverty related diseases that occur at an old age which include: diarrhea, parasite infestations, respiratory infections, immunizable childhood infections, eye infections and malnutrition. These countries are at the same time facing steady increase of diabetes, CVA, rheumatic conditions and cancer

Communicable’ diseases are divided into 

  • Contact contagious diseases
  • STDs and HIV/AIDs
  • Vector borne diseases
  • Diseases related to contaminated water and food
  • Airborne diseases
  • Blood borne diseases
  • Diseases from the animals and their products
  • Helminthic diseases

Some Communicable diseases and there causative agents.

Causative Organism Disease/Infection
Rabies virus Rabies
Influenza A virus Avian influenza (Bird flu)
Vibrio cholerae Cholera
Plasmodium species Malaria
Severe acute respiratory syndrome coronavirus (SARS-CoV) Severe acute respiratory syndrome (SARS)
Trypanosoma species Trypanosomiasis
Tsetse fly Sleeping sickness (African trypanosomiasis)
Wuchereria bancrofti Elephantiasis
Dracunculus medinensis Guinea worm disease
Rotavirus Diarrhea (caused by rotavirus)
Mumps virus Mumps
Human immunodeficiency virus (HIV) HIV/AIDS
Varicella-zoster virus Chickenpox
Measles virus Measles
Yellow fever virus Yellow fever
Arboviruses Arboviral diseases
African swine fever virus African swine fever
Brucella species Brucellosis
Salmonella enterica serovar Typhi Typhoid fever
Schistosoma species Schistosomiasis
Poliovirus Poliomyelitis
Shigella species Dysentery
Bacillus anthracis Anthrax

Why are communicable diseases important in Africa?  

  1. Many of them are very common 
  2. Some of them are very serious and cause death and disability 
  3. Some of them cause widespread outbreaks of the disease- epidemics 4. Many of them are preventable by fairly simple means 
  4. Many are particularly serious and more common in infants and children. 

Organisms and agents of disease 

The living organisms that cause communicable diseases are of different sizes and sorts. The largest, like tape or filarial worms are visible to the eyes. They are made of many cells and  are called metazoa.  

Complicated but single celled organisms like malaria parasites and amoeba are called protozoa.  They are smaller and can only be seen when magnified with a microscope. Smaller still are bacteria which are simple, single cell, best seen under a microscope after they  have been stained with dyes. 

Rickettsiae and chlamydiae are smaller and can only multiply within cells. Smallest of all are the viruses. These cannot be seen with an ordinary microscope.

Epidemiological Triad

Patterns of communicable diseases 

Different diseases are common in different places and different times. To understand why this  happens we need to consider the living organisms of disease- the agent; the people they infect the host and the surrounding in which they live- the environment

The agents need a suitable environment in which to grow and multiply and must be able to  spread and infect other hosts. If they do not succeed in doing this, they die out.

There is therefore a balance between the agent, the host and the environment which can change  and be made to change in different ways.


Hosts (people) are affected by environment, for example, they may live in a hot climate in which  there many mosquitoes. But people can also change this environment by draining swamps,  changing the vegetation and adding competing hosts such as animals. 

Similarly, the environment can affect the agent, for example, the altitude and the temperature  for malaria. 



Infectious disease 

An infectious disease is an illness due to a specific infectious agent or its toxic products that arise  through a transmission of that agent or its products from an infected person, animal or reservoir  to a susceptible host, either directly through an intermediate plant or animal host, vector or inanimate environment. 


Infection is the entry and development of an infectious agent in the body accompanied by an  immune response. 


Manifestation of infection through symptoms and signs 


Someone who has met with an infectious agent in a way that is known to cause disease 


Colonization is the presence of a replicating microorganism without clinical or subclinical  infection or disease. No immune response. 


Carrier is a person that harbours a specific infectious agent in the absence of clinical disease and  serves as a potential source of infection.


The reservoir of infection is the animal or place in which a particular organism usually lives and  multiplies. Most of the important communicable diseases humans are the main reservoirs. 

Route of transmission 

The route of transmission is the way in which an organism leaves the infected host or source and  travels to a new susceptible person. 


The source of infection is the animal or place from which the particular organism spreads to its  new host. 

Incubation period 

The incubation period is the time between infection and the appearance of signs and symptoms  of illness. 


Epidemiology is the study of the distribution and patterns of health events, health characteristics  and their causes or influences in a well defined population. Or 

It is a branch of medicine that deals with the study of the causes, distribution and control of  diseases in the population. 


It means the disease is present in the community at all times but in a relatively low frequency Something that is endemic is typically restricted or peculiar to a locality or region. For example  malaria is endemic in some areas of Africa. 


An epidemic is a sudden severe outbreak of an infectious disease that spreads rapidly within a  region or group, affecting a large proportion of people. 


A pandemic occurs when an epidemic becomes widespread and affects a whole region, a  continent or the entire world. 

Clinical disease 

A clinical disease is a disease which has physical manifestations (clinical signs and symptoms). 

Susceptible host 

A susceptible is someone that is exposed to an infectious disease. 


A vector is an animal, usually an insect that transmits parasitic microorganisms from person to  person or from infected animals to human beings. 


Transmission cycle 

The transmission cycle describes how organisms grow, multiply and spread. In some cases humans may be the only host, in which case the infection spreads directly from  person to person, e.g. measles. In other cases, humans are the final hosts from whom the  organism has no chance to pass further, e.g. tetanus. 


There are three parts of a transmission cycle for an agent or organism: 

Source —-> Transmission  —-> Susceptible Host 


The source of an infection can be an infected person or animal, or soil. People and animals may  have clinical disease, subclinical disease or be carriers. 


The main routes of transmission are: 

  • ❖ Direct contact (skin, mucous membrane, sexual intercourse) 
  • ❖ Vector transmission 
  • ❖ Fecal contamination of soil, food and water which are ingested. 
  • ❖ Contact with animals or their products (e.g. biting) 
  • ❖ Airborne transmission (inhalation) 
  • ❖ Transplacental (mother to child) transmission 
  • ❖ Blood contact (injections, surgery, blood transfusion) 

Susceptible Host 

A susceptible host is one with low resistance to a particular infection. Low resistance may be due  to: 

  •  Not having met the organism before and therefore not having any immunity to it. For  example, at the age of 6-12 months, a child loses the passive immunity against measles  which was acquired from the mother during pregnancy. When in contact with another child who has measles, the child will develop the disease because of no immunity against  measles 
  •  Having another serious illness like AIDS at the same time. Such people have a higher risk  of developing tuberculosis. 
  •  Malnutrition which can make the infection worse. 

Principles of communicable disease control and prevention 

The aim of control is to tip the balance against the agent. This may be done by: 

  1. Attacking the source 
  2.  Interrupting route of transmission 
  3.  Protecting the host 

Attacking the Source 

Interrupting Transmission 

Protecting the Host


Environmental sanitation 



Personal hygiene 


Reservoir control 

Vector control 

Personal protection


Disinfection and sterilization 

Better nutrition 

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