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Epidemiology And BioStatistics Notes Za Wizara Cmt Nta 4 Clinical Medicine First Year

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Natural History and Levels of Prevention of Diseases

Natural History of Disease

Refers to the course of a disease over time, when unaffected by any human intervention like prevention, treatment or rehabilitation.



Stages of the Natural History of Disease

i. This is a time before the onset of a disease in the human population; however, the factors responsible or that favour its occurrence already exist in that population. (i.e., the groundwork has been laid.)

ii. In this period, there is interaction between host, agent and environmental factors.

iii. This interaction or interplay ultimately produces an opportunity for the start of a disease, or for infection (in the case of an infectious disease). (Also known as ‘disease stimulus’.)



i. This is the time between infection up to the development of signs and symptoms

ii. This disease process period can be terminated or shortened by human interventions in terms of treatment (or secondary prevention


i. This is the stage where the agent has already been removed from the affected patients but there is the effect of the disease persisting in the form of disability (or sequel of the disease).

ii. A new disease may be a post-pathogenesis of another disease, e.g. rheumatic fever is a post pathogenesis of streptococci sore throat.


Levels of Disease Prevention

There are four levels of prevention have been identified in epidemiology:

i. Primordial Prevention

ii. Primary Prevention

iii. Secondary Prevention

iv. Tertiary Prevention


Primordial Prevention

Primordial prevention: Preventing agents or risk factors; preventing the imbalance of interaction between host, agent and environmental factors, so that disease may not occur.


Efforts and interventions at the primordial prevention level involve anticipation of disease occurrence and modification of the conditions responsible for the occurrence, before disease happens.

Examples of primordial prevention interventions include:

Policy and public health interventions that discourage, limit, and/or prohibit cigarette smoking. Cigarette smoking can lead to high blood pressure, strokes, or lung cancer.




Secondary Prevention

Identifying/detecting individuals who are already infected with a given disease as early as possible, in order to stop the disease from spreading/developing further.

i. Infected individuals should be diagnosed and treated as early as possible, to increase recovery rates and reduce disability, morbidity, and mortality rates.

ii. Screening for early diagnosis and treatment can be done for sub-clinical diseases using laboratory tests.

iii. Clinical examination can be done to discover early manifestation of disease, which is easier to reverse.

iv. Treatment can be provided via drugs, lifestyle modification, or by natural remedies.



Tertiary Prevention

Tertiary prevention: preventing further disability, or preventing/postponing death due to disability or secondary disease(s).

Sometimes, the agent has been removed while the disability or effect of the disease is still visible or felt.

i. Tertiary prevention may refer to patients who have been cured of a primary disease but have a permanent disability (e.g. post-polio paralysis or post-trachoma blindness) and/or need rehabilitation.

ii. Examples of tertiary prevention include:

iii. Palliative care for patients with AIDS or cancer


The Role and Characteristics of Infectious Agents in Disease Transmission


Components of Disease Transmission
Transmission of diseases in humans requires the following components:

i. An agent, capable of infecting a human

ii. A source (an infected host or reservoir of infection)

iii. A portal of exit from the source

iv. A suitable means of transmission

v. A portal of entry into a new, susceptible host




The etiological factor which must be present for the disease, disability or pathological state to occur in a susceptible host.

An agent may be defined as the presence, absence, excess or deficiency of a certain factor.


Characteristics of Infectious Agents


i. Refers to the ability of the agent to survive under adverse environmental conditions.  

ii. Some agents are remarkably resistant, such as Mycobacterium tuberculosis. It can survive in a presence of alcohol, acid, clostridium tetani, etc.

iii. Others are extremely fragile, such as gonococci and influenza viruses.  They cannot survive for a long time in sunlight.


i. The capacity of a micro-organism to enter a susceptible host and cause infection.

ii. It can be expressed as the proportion of the susceptible population which is infected by a particular organism.

iii. Experimentally it can be thought of as the minimum number of particles, pathogens or agents necessary to cause infection in 50% of a group of hosts of the same species. (This is also called the ‘infective dose 50%,’ or ID50.)



i. Refers to the capacity of a micro-organism to produce signs and symptoms in the infected host.

ii. It can be expressed as the proportion of the infected population that develops signs and symptoms.


i. Refers to the ability of the agent to induce antibody production in the host.  These antibodies may not necessarily be protective.



i. Refers to the capacity of the agent to produce a toxin or poison that can cause pathogenic effects on the host. The pathogenic effect of agents in diseases such as botulism and shellfish poisoning depends on the toxin produced by the micro-organism rather than on the direct effect of the micro-organism itself.



i. Refers to the severity of the disease. It is the degree of pathogenicity of an infectious agent. One measure of virulence is the Case Fatality Rate (CFR)



An infected host or reservoir of infection.

Many diseases are transmissible even during periods when the disease is not (yet) clinically manifest. For example, mumps can be transmitted during the incubation period, and gonorrhoea is transmissible in asymptomatic cases.

Some diseases can be transmitted years after recovery, as in a chronic carrier state (as with typhoid and hepatitis B), or by reactivation of a latent infection (as with herpes).


Portal of Exit in the Human Host

i. Respiratory passages

ii. Alimentary canal

iii. Openings in the genito-urinary system

iv. Skin lesions

v. Additional portals of exit may be made available through:

vi. Insect bites

vii. Drawing of blood

viii. Surgical procedures

ix. Accidents and other injuries



Refers to any human beings, animals, arthropods, plants, soil or a combination of these in which an infectious agent normally lives and multiplies, and on which it primarily depends for survival and reproduction in such a manner that it can be transmitted to a susceptible host.

A reservoir of infection may also be termed as the ‘natural habitat’ of the infectious agent.


Different Means of Disease Transmission

Direct transmission means that the agent is transmitted directly from the infected host (man or animal) to the new host (e.g. influenza, gonorrhoea, etc.) Direct transmission may be horizontal or vertical.


Horizontal is the transmission of an infectious agent, such as bacterial, fungal, or viral infection, between members of the same species that are not in a parent-child relationship. Examples of horizontal transmission of diseases include: Droplet infection, Faeco-oral route, Genital and Direct skin contact

a. While vertical transmission is transmission from mother to child, often in utero or during childbirth (also referred to as perinatal infection).

b. Examples of vertical transmission of diseases include:

i. Trans placental and through Genital tract

i. Mother-to-child transmission (MTCT) may occur in cases such as HIV, hepatitis and syphilis.


Indirect Transmission

Indirect transmission requires a vehicle or vector to carry the agent of disease from one host to another.


Substances such as water, air, food, blood used in transfusion, or formites (inanimate objects used by an infectious host, such as clothing, handkerchiefs, doorknobs etc.)

Multiplication may or may not take place in or on the vehicle (e.g. in influenza, hepatitis, and streptococcal disease)


Vectors may be either mechanical or biological, but are always living.  

Mechanical vectors are such animals and insects, which can carry agents from place to place on their feet, proboscis, or other body parts

For example, flies are vectors of Shigellosis. Flies can breed in infected feces, and then contaminate food, which humans may ingest.   

Biological vectors must have growth or multiplication of organisms occurring within the body of the vector

For example, fleas infected with Yersinia pestis bacteria transmit plague to humans and other mammals through their bites.


The Role of Host in Disease Transmission

Host refers to the organism capable of being infected by the agent

Obligate host

If a human body is necessary for the life cycle or for the continued existence of an agent, the human constitutes the obligate host for this agent.

Humans are the obligate hosts for such agents as malaria (plasmodium species).


Incidental hosts (occasional or accidental)

Hosts that are not usually involved in the natural cycle of transmission.

Humans are incidental hosts for diseases like measles, salmonella typhi, etc.


Characteristics of the Host


An infected person harbours an infectious agent and has either manifest disease or an asymptomatic infection.

They may or may not be infectious to others.



A person from whom the infectious agent can be naturally acquired.

A person or their articles or clothing may also be merely contaminated with an infectious agent, without being infected.


A person who possesses specific protective antibodies or cellular immunity as a result of previous infection or immunisation.

Immunity is relative: an ordinarily effective protection may be overwhelmed by an excessive dose of the agent.  

It may also be impaired by immune-suppressive drug therapy or concurrent disease (such as AIDS).


Inherent resistance

The ability to resist disease, independent of antibodies or specifically developed tissue response.

It commonly resides in anatomical or physiological characteristics of the host, and may be genetic or acquired, permanent or temporary.



A person is considered susceptible if they do not possess sufficient resistance (inherent and/or acquired) against a particular pathogenic agent to prevent contracting a disease when exposed to the agent.

Susceptibility of a host may be modified by characteristics, such as age, sex, race, genetic make-up, physiological state, habits and customs, pathological state and previous experience with the agent (immunity).

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