A communicable disease is caused by pathogens that can be transmitted between humans and animals. Common pathogens include viruses, bacteria, and fungi. Communicable diseases spread through various routes such as physical contact, airborne droplets, contaminated fluids or foods, or bites from insects or animals. Factors that influence the transmission of communicable diseases include the infectious agent, the route of exit from the host, the mode of transmission, the route of entry into a new host, and characteristics of the susceptible host. Communicable diseases have historically been a major cause of death but advances in public health have helped reduce their spread.

1. Communicable disease
A communicable disease is a disease that spreads from one person or animal to another.
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Pathogens such as viruses, bacteria, and fungi cause these diseases.
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a disease due to an organism such as bacteria or a virus that is transmitted person-to-person or from animals or
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the physical environment to humans by a variety of routes, including from air and water, contaminated
considered a subset of infectious disease.
infectious disease;
a disease caused by an organism such as bacteria or a virus. here used to include communicable diseases as well
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as other infections that are not communicable.
What Is the Burden of Disease Caused by Communicable Diseases?
Communicable disease may be caused by a wide variety of organisms, such as bacteria, viruses, or parasites
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For many centuries, communicable diseases were the leading cause of death and disability among all ages, but
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especially among the young and the old
May cause great epidemics
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Can also become endemic and become routine causes of death
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Aspects of communicable disease
A communicable disease is any disease that passes between people or animals.
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People sometimes refer to communicable diseases as “infectious” or “transmissible” diseases.
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Pathogens, including bacteria, viruses, fungi, and protists, cause communicable diseases.
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A person may develop a communicable disease after becoming infected by the pathogen.
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direct contact with a person carrying the pathogen
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contact with contaminated fluids, such as blood, mucus, or saliva
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inhaling contaminated droplets from another person’s cough or sneeze
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receiving a bite from an animal or insect carrying the pathogen
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consuming contaminated water or foods
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How do these communicable diseases spread?
How these diseases spread depends on the specific disease or infectious agent.
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1. physical contact with an infected person, such as through touch (staphylococcus), sexual intercourse
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(gonorrhea, HIV), fecal/oral transmission (hepatitis A), or droplets (influenza, TB).
2. contact with a contaminated surface or object (Norwalk virus), food (salmonella, E. coli), blood (HIV,
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hepatitis B), or water (cholera);
3. bites from insects or animals capable of transmitting the disease (mosquito: malaria and yellow fever; flea:
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plague); and
4. travel through the air, such as tuberculosis or measles.
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Factors involved in the transmission of communicable diseases
Transmission; pathogen transferred directly from one individual to another.
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Six major factors can be identified: the infectious agent, the reservoir, the route of exit, the mode of
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transmission, the route of entry and the susceptible host.
1) Infectious agents
caused by the entrance and reproduction of an infectious agent in a body.
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The groups of organisms that cause infectious diseases are categorized as bacteria, viruses, fungi, and parasites.
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2) Route of exit
Before an infectious agent can be transmitted to other people, it must first get out of the infected host.
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The site on the infected host through which the infectious agent gets out is called the route of exit.
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2. 3) Respiratory tract
The routes of exit from the respiratory tract are the nose and the mouth.
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Some infectious agents get out of the infected host in droplets expelled during coughing, sneezing, spitting or
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talking, and then get transmitted to others.
4) Gastrointestinal tract
The anus is the route of exit from the gastrointestinal tract (or gut). Some infectious agents leave the human
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body in the stool or faeces.
For example, the infectious agents of shigellosis, a disease which can cause bloody diarrhoea, use this route of
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exit.
5) Skin
Some types of infectious agents can exit the body through breaks in the skin.
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For example, this route of exit is used by Plasmodium protozoa, which are present in the blood and get out of
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the human body when a mosquito bites through the skin to suck blood.
6) Direct modes of transmission
Direct transmission refers to the transfer of an infectious agent from an infected host to a new host, without the
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need for intermediates such as air, food, water or other animals.
a) Person to person: The infectious agent is spread by direct contact between people through touching,
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biting, kissing, sexual intercourse.
b) Transplacental transmission: This refers to the transmission of an infectious agent from a pregnant
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woman to her fetus through the placenta. An example is mother-to-child transmission (MTCT) of HIV.
7) Indirect transmission
Indirect transmission is when infectious agents are transmitted to new hosts through intermediates such as air,
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food, water, objects or substances in the environment, or other animals.
a) Airborne transmission: The infectious agent may be transmitted in dried secretions from the
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respiratory tract, which can remain suspended in the air for some time.
b) Vehicle-borne transmission: A vehicle is any non-living substance or object that can be
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contaminated by an infectious agent, which then transmits it to a new host. Contamination refers to the presence
of an infectious agent in or on the vehicle.
c) Vector-borne transmission: A vector is an organism, usually an arthropod, which transmits an
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infectious agent to a new host. Arthropods which act as vectors include houseflies, mosquitoes, lice and ticks.
Stage of exposure
In the stage of exposure, the susceptible host has come into close contact with the infectious agent, but it has not
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yet entered the host’s body cells.
Examples of an exposed host include:
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a person who shakes hands with someone suffering from a common cold.
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a child living in the same room as an adult with tuberculosis
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a person eating contaminated food or drinking contaminated water.
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Stage of infection
At this stage the infectious agent has entered the host’s body and has begun multiplying.
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The entry and multiplication of an infectious agent inside the host is known as the stage of infection.
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e.g. headache, vomiting, dizziness
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Stage of infectious disease
At this stage the clinical manifestations of the disease are present in the infected host. For example, a person
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infected with Plasmodium falciparum, who has fever, vomiting and headache, is in the stage of infectious

3. disease
Susceptible hosts and risk factors
After an infectious agent gets inside the body it has to multiply in order to cause the disease.
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In some hosts, infection leads to the disease developing, but in others it does not.
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diseases like HIV/AIDS which suppress immunity
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