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Unit Code: MPH 314
Unit Title: Communicable Diseases
Contact Hours: 45
Pre-Requisite: NONE
Purpose of the Course
This unit is designed to enable the student to acquire knowledge and skills for the prevention and
control of communicable diseases.

Expected Learning Outcomes of the Course
By the end of the course unit the learners should be able to: -
1. Define and describe the concepts and classification of communicable diseases.
2. Describe etiology and epidemiology of communicable diseases in the tropics
3. Discuss emerging and re-emerging diseases. Discuss sexually transmitted infections
including HIV and AIDS
4. Discuss the strategies for prevention and control of communicable diseases

Course Content
Communicable diseases: concepts, definition, etiology, epidemiology, mode of transmission,
incubation, diagnosis, clinical signs and symptoms, treatment. Classification: mode of
transmission, etiology, habitat.
Etiology and epidemiology of communicable disease. Common tropical communicable
diseases: distribution; etiology; risk factors; signs and symptoms; diagnosis; treatment; prevention
and control. Emerging and re-emerging diseases: Sexually transmitted infections including HIV
/ AIDS. Strategies: control; isolation, quarantine, disinfection, decontamination, surveillance,
personal hygiene, treatment, vector, control.
Prevention: immunization, health education, nutrition, safety measures, environmental
manipulation, safe water, sanitation, law enforcement, sanitary, disposal of the dead. Impact:
resources, health of the relatives.

Method of Delivery
Lectures, group discussion, Assignments, field trips, practicals.

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Instructional Materials and Equipment
Journals; scientific papers; Text books; facts and figures,

Course Assessment
Continuous Assessments Tests (2 Sit-in Tests (20 %), Assignments (10 %) 40%
End of Semester Examinations 60%
Total 100%

Core Reading Materials for the Course
1. Coen, Jonathan; Powderly, William G. (2004). Infectious diseases. Mosby, Edinburg
2. Feigin, Ralph D. (et al) ed., (2004). Textbook of Pediatric infectious disease (5th ed.).
Elsevier, Philadelphia.
3. Giesecke, Johan (2002). Modern infectious disease epidemiology. Arnold, London.
4. Bartlett, John G. (2004). Pocket book of infectious disease therapy. Lippincot Williams and
Wilkins, Philadelphia.

Recommended reading materials
1. Smolinski, mark S.; Hamburg, Margaret A.; Lederberg, Joshua (2003). Microbial threats
to health: emergence, detection and response. National Academy press, Washington DC.
2. Collinge, Shaon K., ed.; Ray chris ed. (2006). Disease ecology: Community structure and
pathogen dynamics. Oxford University press, New York.
3. Karani, Ann K. (1998). Common communicable and tropical diseases: a handbook for
nurses. Magayu K. Magayu media congress, Nairobi
4. Nordberg, Erick, ed., Carte, Jane …(et al) (1999). Communicable disease: a manual for
health workers in Sub-Saharan Africa. AMREF, Nairobi.

Journals
1. Control of Communicable Diseases Manualbooks.google.co.ke › books David L.
Heymann, American Public Health Association · 2008
2. Communicable Disease Control and Health Protection Handbookbooks.google.co.ke ›
books
Jeremy Hawker, Norman Begg, Iain Blair · 2012

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INTRODUCTION TO COMMUNICABLE DISEASES
-Concepts;
-Definition,
-Etiology,
-Epidemiology,
-Mode of transmission,
-Incubation,
-Diagnosis,
-Clinical signs and symptoms,
-Treatment.
-Classification; mode of transmission, etiology, habitat.

Definitions
Communicable diseases
-The term disease refers to conditions that impair normal tissue function.
-An infection results when a pathogen invades and begins growing within a host.
-Disease results only if and when, as a consequence of the invasion and growth of a pathogen,
tissue function is impaired.
-Communicable diseases are diseases caused by microorganisms that may be transmitted directly
or indirectly from man to man, or from animals to man.
-Infectious disease is synonymous with communicable disease, and means any disease caused by
vegetable or animal microorganisms that is capable of being transmitted by infection, with or
without contact.
-Contagious diseases are those that are spread from person to person, or from the sick to the well,
by direct or indirect contact, either by intimate personal contact with a patient or through contact
with his secretions or with an object recently contaminated by him.
-All contagious and infectious diseases are communicable diseases, but many infectious diseases
are not contagious.
-Examples of non-contagious infectious diseases are malaria, typhus fever, and other afflictions
that are spread only by the bites of insects of certain species.

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Etiology
-Etiology refers to the study of the causes and origins of diseases, including communicable
diseases.
-Understanding the etiology of these diseases is crucial for developing effective prevention and
control strategies.
-Communicable diseases are caused by a variety of pathogens, including:
 Bacteria: Examples include Mycobacterium tuberculosis (tuberculosis) and Salmonella.
 Viruses: Examples include HIV, influenza viruses, and hepatitis viruses.
 Fungi: Examples include those causing ringworm and candidiasis.
 Parasites: Examples include Plasmodium (malaria) and intestinal parasites.
 Prions: These are misfolded proteins that can cause rare but fatal neurodegenerative
diseases.
-These pathogens can be transmitted through various means such as direct contact or indirect
contact.

Epidemiology
-Epidemiology is derived from three Greek root words: epi – on, upon demos – people, logo –
study
-Epidemiology is, thus, the study of what is upon the people. In modern terms, it is the science of
the distribution of disease and its determinants (causes).
-Epidemiology is the quantitative study of the occurrence of disease and factors that influence
disease frequency and distribution.


Transmission
-Transmission is a process in which several events happen one after the other in the form of a
chain. Hence, this process is known as a chain of transmission.
-Six major factors can be identified: the infectious agent, the reservoir, the route of exit, the mode
of transmission, the route of entry and the susceptible host.

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Infectious agents
-Helminths are worms made up of many cells; for example, Ascaris lumbricoides.
-Protozoa are micro-organisms made up of one cell; for example, Plasmodium falciparum.
-Bacteria are also micro-organisms made up of one cell, but they are much smaller than protozoa
and have a different structure; for example, Vibrio cholerae, which causes cholera.
-Viruses are infectious agents that do not have the structure of a cell. They are more like tiny boxes
or particles and are much smaller than bacteria; for example, HIV (the Human Immunodeficiency
Virus), which can lead to AIDS.
Reservoirs of infectious agents

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-Many infectious agents can survive in different organisms, or on non-living objects, or in the
environment.
-Some can only persist and multiply inside human beings, whereas others can survive in other
animals, or in soil or water.
-The place where the infectious agent is normally present before infecting a new human is called
a reservoir.
-Without reservoirs, infectious agents could not survive and hence could not be transmitted to other
people.
-Humans and animals which serve as reservoirs for infectious agents are known as infected hosts.
Two examples are people infected with HIV and with the bacteria that cause tuberculosis; these
infectious agents persist and multiply in the infected hosts and can be directly transmitted to new
hosts.
-Animals can also be reservoirs for the infectious agents of some communicable diseases. For
example, dogs are a reservoir for the virus that causes rabies.
-Diseases such as rabies, where the infectious agents can be transmitted from animal hosts to
susceptible humans, are called zoonoses (singular, zoonosis).
-Non-living things like water, food and soil can also be reservoirs for infectious agents, but they
are called vehicles (not infected hosts) because they are not alive.
Route (portal) of exit
-Before an infectious agent can be transmitted to other people, it must first get out of the infected
host.
-The site on the infected host through which the infectious agent gets out is called the route of exit.
-Some common examples are:

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i) Respiratory tract
-The routes of exit from the respiratory tract are the nose and the mouth.
-Some infectious agents get out of the infected host in droplets expelled during coughing, sneezing,
spitting or talking, and then get transmitted to others.
-For example, people with tuberculosis in their lungs usually have a persistent
cough; Mycobacterium tuberculosis uses this as its route of exit.
ii) Gastrointestinal tract
-The anus is the route of exit from the gastrointestinal tract (or gut).
-Some infectious agents leave the human body in the stool or faeces.
-For example, the infectious agents of shigellosis, a disease which can cause bloody diarrhea, use
this route of exit.
iii) Skin
-Some types of infectious agents can exit the body through breaks in the skin.
-For example, this route of exit is used by Plasmodium protozoa, which are present in the blood
and get out of the human body when a mosquito bites through the skin to suck blood.
Modes of transmission
-Once an infectious agent leaves a reservoir, it must get transmitted to a new host if it is to multiply
and cause disease.
-The route by which an infectious agent is transmitted from a reservoir to another host is called
the mode of transmission.
-It is important to identify different modes of transmission, because prevention and control
measures differ depending on the type.
-Modes of transmission could be direct or indirect.

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Direct transmission refers to the transfer of an infectious agent from an infected host to a new
host, without the need for intermediates such as air, food, water or other animals.
-Direct modes of transmission can occur in two main ways:
 Person to person: The infectious agent is spread by direct contact between people
through touching, biting, kissing, sexual intercourse or direct projection of respiratory
droplets into another person’s nose or mouth during coughing, sneezing or talking. A
familiar example is the transmission of HIV from an infected person to others through
sexual intercourse.
 Transplacental transmission: This refers to the transmission of an infectious agent
from a pregnant woman to her fetus through the placenta. An example is mother-to-
child transmission (MTCT) of HIV.

Indirect transmission is when infectious agents are transmitted to new hosts through
intermediates such as air, food, water, objects or substances in the environment, or other animals.
Indirect transmission has three subtypes:
 Airborne transmission: The infectious agent may be transmitted in dried secretions
from the respiratory tract, which can remain suspended in the air for some time. For
example, the infectious agent causing tuberculosis can enter a new host through
airborne transmission.
 Vehicle-borne transmission: A vehicle is any non-living substance or object that can
be 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.
 Vector-borne transmission: A vector is an organism, usually an arthropod, which
transmits an infectious agent to a new host. Arthropods which act as vectors include
houseflies, mosquitoes, lice and ticks.
Arthropods are invertebrates (animals without backbones), such as insects, which have segmented
bodies and three pairs of jointed legs.

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Route (port) of entry
-Successful transmission of the infectious agent requires it to enter the host through a specific part
of the body before it can cause disease.
-The site through which an infectious agent enters the host is called the route of entry.
- The routes of entry are:
o The respiratory tract: some infectious agents enter the body in air breathed into
the lungs. Example: Mycobacterium tuberculosis.
o The gastrointestinal tract: some infectious agents enter through the mouth.
Example: the infectious agents causing diarrheal diseases enter through the
mouth in contaminated food, water or on unclean hands.
o The skin provides a natural barrier against entry of many infectious agents, but
some can enter through breaks in the skin. Example: malaria parasites
(Plasmoduim species) get into the body when an infected mosquito bites through
the skin to suck blood.

Susceptible hosts
-After an infectious agent gets inside the body it has to multiply in order to cause the disease.
-In some hosts, infection leads to the disease developing, but in others it does not.
-Individuals who are likely to develop a communicable disease after exposure to the infectious
agents are called susceptible hosts.
-Different individuals are not equally susceptible to infection, for a variety of reasons.
-Factors that increase the susceptibility of a host to the development of a communicable disease
are called risk factors.

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-Some risk factors arise from outside the individual – for example, poor personal hygiene, or poor
control of reservoirs of infection in the environment.
-Factors such as these increase the exposure of susceptible hosts to infectious agents, which makes
the disease more likely to develop.
-Additionally, some people in a community are more likely to develop the disease than others,
even though they all have the same exposure to infectious agents.
-This is due to a low level of immunity within the more susceptible individuals.
-Immunity refers to the resistance of an individual to communicable diseases, because their white
blood cells and antibodies (defensive proteins) are able to fight the infectious agents successfully.
-Low levels of immunity could be due to:
 diseases like HIV/AIDS which suppress immunity
 poorly developed or immature immunity, such as in very young children
 not being vaccinated
 poor nutritional status (e.g. malnourished children)
 pregnancy.

Incubation
-The incubation period is defined as the time from exposure to onset of disease, i.e., it corresponds
to the time from infection with a microorganism to symptom development.
-Different pathogens have unique replication rates and mechanisms, affecting the incubation
period.

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-The way a pathogen enters the body (e.g., ingestion, inhalation, skin contact) can influence the
incubation period. The incubation period and severity of a disease can be influenced by the amount
of pathogen exposure.
-Understanding this period is essential for diagnosing, managing, and controlling the spread of
diseases. This period varies depending on several factors, including the type of pathogen, the route
of infection, the host’s immune response, and individual characteristics.
-The length of the incubation period depends on the disease and the infectious agent.
-It can be very short, perhaps only several days in the case of a streptococcal sore throat, or perhaps
several weeks in the case of smallpox, or perhaps a decade in the case of the acquired immune
deficiency syndrome (AIDS).
-After an individual is exposed to an infectious agent, the agent multiplies, and the host defenses
are weakened.
-Eventually, the individual may experience the onset of clinical disease.
-Individuals may or may not be infectious (i.e., capable of transmitting the infection to others)
during the incubation period or subsequently. The infectious period, the period when individuals
are capable of transmitting the infection to others, does not necessarily coincide with the incubation
period.
-For example, the infectious periods for smallpox and Ebola cases do not generally begin until
after the onset of symptoms.
Importance of incubation periods
The incubation period is important for several reasons:
-Diagnosis and early detection: Knowing the typical incubation period helps healthcare providers
identify the likely timing of exposure and diagnose infections based on symptom appearance. In

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some cases, knowledge of incubation periods can guide screening strategies, particularly for
diseases with long or variable incubation periods.
-Infection control and prevention: Accurate knowledge of incubation periods assists in determining
the appropriate isolation or quarantine duration to prevent further spread. For diseases with specific
incubation periods, vaccination schedules can be optimized to maximize effectiveness and
minimize disease transmission.
-Public health response: During an outbreak, understanding incubation periods helps trace the
source of infection and identify contacts who may need monitoring or vaccination. Public health
authorities use incubation period data to assess risks and implement control measures, such as
travel advisories or targeted vaccination campaigns.
-The variability in incubation periods among individuals can complicate diagnosis and
management, as symptoms may appear at different times. Some infections may lead to
asymptomatic cases, making it challenging to identify and manage cases based on symptom onset
alone.
-For newly emerging diseases, such as those caused by novel pathogens, incubation period data
may be limited or evolving, requiring ongoing research and surveillance.

Diagnosis
-Diagnosis generally refers to the identification of a disease, condition, or injury based on its signs
and symptoms.
-It involves a process of investigation, which may include reviewing a patient's medical history,
conducting a physical examination, and potentially using various tests like blood work or imaging.
-Ultimately, a diagnosis is a decision reached by a healthcare professional after carefully evaluating
the gathered information, aiming to understand the nature and cause of the patient's health issue.

 Identification of Disease:

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The core meaning of diagnosis is the act of recognizing and naming a specific disease or
condition.
 Investigative Process:
It's not just about guessing; diagnosis involves a systematic approach, including
gathering information through various means like:
 Medical History: Past illnesses, family history, and current symptoms.
 Physical Examination: Assessing a patient's physical state.
 Diagnostic Tests: Blood tests, imaging (X-rays, MRIs, etc.), and other specialized
tests.

 Decision-Making:
The collected information is then analyzed to determine the most likely explanation for
the patient's condition, leading to a specific diagnosis.
 Beyond Symptoms:
While symptoms are crucial, a diagnosis can also involve understanding the underlying
causes (etiology) of a disease, which can be helpful for treatment and prevention.
 Importance:
Diagnosis is a critical step in healthcare as it guides treatment decisions, helps predict the
likely outcome of a condition, and aids in preventing future occurrences.

Types of medical diagnosis are:
 Provisional: Based on physical exam and clinical findings.
 Histopathological: Done by a pathologist after examining sample tissue under a
microscope.
 Final: Done based on provisional diagnosis and investigations.

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The importance of accurate diagnosis
-Accurate diagnosis is paramount for several reasons.
-Firstly, it guides appropriate and timely treatment decisions. Proper identification of a condition
allows healthcare providers to implement tailored interventions, optimizing patient outcomes and
minimizing unnecessary medical procedures or treatments.
-Secondly, diagnosis empowers patients with knowledge and understanding, enabling them to
actively participate in their own healthcare journey.
-Moreover, an accurate diagnosis plays a pivotal role in healthcare planning, resource allocation,
and research, contributing to the overall advancement of medical knowledge.
Challenges in the diagnostic process
-The diagnostic process is complex and multifaceted, with various challenges that clinicians must
navigate.
-One of the primary challenges is the sheer diversity of diseases and conditions that can manifest
with similar symptoms. This diagnostic “zebra puzzle” demands a careful evaluation of a patient’s
medical history, physical examination, laboratory tests, and imaging studies to arrive at an accurate
conclusion.
-Additionally, some conditions present with atypical or vague symptoms, requiring clinicians to
maintain a high level of suspicion and employ specialized diagnostic techniques.
-Another challenge arises from the inherent limitations of diagnostic tests. While modern medicine
boasts an impressive array of sophisticated tests, no diagnostic tool is infallible. False-positive and
false-negative results can occur, necessitating a nuanced interpretation of test findings in the
context of a patient’s clinical presentation.
-Moreover, diagnostic tests are not equally accessible worldwide, leading to disparities in
healthcare delivery and potentially impacting diagnostic accuracy.

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Clinical signs and symptoms
-A symptom is a manifestation of disease apparent to the patient himself, while a sign is a
manifestation of disease that the physician perceives.
-The sign is objective evidence of disease; a symptom, subjective.
-Symptoms represent the complaints of the patient, and if severe, they drive him to the doctor's
office. If not severe, they may come to light only after suitable questions.
-Any objective evidence of a disease, such as a skin rash or a cough, is a sign. However, less
obvious breaks in normal function, such as stomachache, lower back pain, and fatigue, are
symptoms and can only be recognized by the person experiencing them.
-Symptoms are subjective, meaning that other people only know about them if informed by the
individual with the condition.
-The key difference between signs and symptoms is who observes the effect.
For example, a rash could be a sign, a symptom, or both:
 If the patient notices the rash, it is a symptom.
 If the doctor, nurse, or anyone other than the patient notices the rash, it is a sign.
 If both the patient and doctor notice the rash, it can be classed as both a sign and a symptom.
-There are three main types of symptoms:
 Remitting symptoms: When symptoms improve or resolve completely, they are known as
remitting symptoms. For examples, symptoms of the common cold may occur for several
days and then resolve without treatment.
 Chronic symptoms: These are long-lasting or recurrent symptoms. Chronic symptoms are
often seen in ongoing conditions, such as
diabetes, asthma, and cancer.

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 Relapsing symptoms: These are symptoms that have occurred in the past, resolved, and
then returned. For instance, symptoms of depression may not occur for years at a time but
can then return.

Signs fit into the following categories:
 Prognostic signs: these are signs that point to the future. Rather than indicating the nature
of the disease, they predict the outcome for the patient, such as what is likely to happen to
them and how severe the disease will probably be.
 Anamnestic signs: These signs point to parts of a person’s medical history. For instance,
skin scars may be evidence of severe
acne in the past.
 Diagnostic signs: These signs help the doctor recognize and identify a current health
problem. For example, high levels of prostate-specific antigen (PSA) in the blood of a male
may be a sign of prostate cancer or a prostate problem.
 Pathognomonic signs: This means that a doctor can link a sign to a condition with full
certainty. For example, the presence of a certain microbe in a blood sample can point to a
specific viral infection.

Treatment
-In a general context, treatment refers to the management and care of a patient to combat disease
or disorder.
-This can include various medical interventions like medications, surgeries, and therapies.
-In the context of substance abuse, treatment involves a broad range of services aimed at addressing
alcohol or drug-related problems.
-General medical context:

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 Management and care: Treatment involves actions taken to alleviate, manage, or cure
health conditions and diseases.
 Medical interventions: This can include medications, therapies, surgeries, and other
procedures.
Substance Abuse Context:
 Broad range of services: Treatment includes diagnostic assessment, counseling, medical,
psychiatric, and social services.
 Addressing problems: It aims to help individuals overcome alcohol or drug-related
problems.

Key aspects of treatment, regardless of the context, often include:
 Diagnosis: Identifying the specific condition or problem.
 Intervention: Applying the chosen treatment approach.
 Monitoring: Tracking progress and making adjustments as needed.
 Evaluation: Assessing the effectiveness of the treatment.

The natural history of a communicable disease
-The natural history of a disease is also referred to as the course of the disease, or its development
and progression.
-The natural history of a communicable disease refers to the sequence of events that happen one
after another, over a period of time, in a person who is not receiving treatment.
-Recognizing these events helps one to understand how particular interventions at different stages
could prevent or control the disease.
-Events that occur in the natural history of a communicable disease are grouped into four stages:
exposure, infection, infectious disease, and outcome.

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Stage of exposure
-Here a contact refers to an association between a susceptible host and a reservoir of infection,
which creates an opportunity for the infectious agents to enter the host.
-In the stage of exposure, the susceptible host has come into close contact with the infectious
agent, but it has not yet entered the host’s body cells.
-Examples of an exposed host include:
 a person who shakes hands with someone suffering from a common cold
 a child living in the same room as an adult with tuberculosis
 a person eating contaminated food or drinking contaminated water.
Stage of infection
-At this stage the infectious agent has entered the host’s body and has begun multiplying.
-The entry and multiplication of an infectious agent inside the host is known as the stage of
infection.
-For instance, a person who has eaten food contaminated with Salmonella typhii (the bacteria that
cause typhoid fever) is said to be exposed; if the bacteria enter the cells lining the intestines and
start multiplying, the person is said to be infected.
At this stage there are no clinical manifestations of the disease, a term referring to the typical
symptoms and signs of that illness.

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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 infected with Plasmodium falciparum, who has fever, vomiting and
headache, is in the stage of infectious disease – in this case, malaria.
-The time interval between the onset (start) of infection and the first appearance of clinical
manifestations of a disease is called the incubation period.
-Not all infected hosts may develop the disease, and among those who do, the severity of the illness
may differ, depending on the level of immunity of the host and the type of infectious agent.
-Infected hosts who have clinical manifestations of the disease are called active cases.
-Individuals who are infected, but who do not have clinical manifestations, are called carriers.
-Carriers and active cases can both transmit the infection to others.
Stage of outcome
-At this stage the disease may result in recovery, disability or death of the patient.
-For example, a child who fully recovers from a diarrheal disease, or is paralyzed from
poliomyelitis, or dies from malaria, is in the stage of outcome.