Dr. Siham Gritly 1

4 community medicineConcept of Disease

Prepared by; Dr. Siham GritlyUniversity of Bhari

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Terms• Disease – a pathological condition of body parts or

tissues characterized by an identifiable group of signs and symptoms.

• Infectious disease – disease caused by an infectious agent such as a bacterium, virus, protozoan, or fungus that can be passed on to others.

• Infection – occurs when an infectious agent or pathogenic organism enters the body and begins to reproduce; may or may not lead to disease.

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• Pathology: scientific study of disease• Pathogen – an infectious agent that causes disease.

• Host – an organism infected by another organism.

• Virulence – the relative ability of an agent to cause rapid and severe disease in a host

• Etiology: causative agent of a disease

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Concept of Disease

• Webster Dictionary “A condition in which body function is impaired, departure from a state of health, an alteration of the human body interrupting the performance of the vital functions

• Oxford English Dictionary – the condition of body or some part of organ of body in which its functions are disrupted or deranged (unbalanced).

• Ecologically – ‘a maladjustment of human organism to the environment’

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Concept of causation

• Germ theory

• Epidemiological Triad

• Multi-factorial causation

• Web of causation

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Germ theory

• the theory stated that all infectious diseases are caused by microorganisms

• • These small organisms, too small to see without

magnification, invade humans, animals, and other living hosts.

• Their growth and reproduction within their hosts can cause a disease.

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Epidemiological triad

• The Epidemiologic Triangle is a model that scientists have developed for studying health problems.

• The Triangle has three corners; agent, host and environment

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Epidemiological triad

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1-Agent, or microbe that causes the disease

• the agent is a microbe—an organism too small to be seen with the naked eye that cause the disease.

• • Disease-causing microbes are;• bacteria, • virus, • fungi, • protozoa

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Bacteria

Bacteria are single-celled organisms. Bacteria have the tools to reproduce themselves, by themselves. They are larger than viruses

They are filled with fluid and may have threadlike structures to move themselves, like a tail.

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Virus

Viruses have no nucleus, cytoplasm, organelles, or cell membrane, so can not carry out cellular functions.Only able to replicate by infecting cells and using the organelles and enzymes very small, size Consists of two parts: a nucleic acid and a protein coat called a capsidNucleic acid may be DNA or RNA but not both.

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HIV virus(human immunodeficiency virus).

AIDS (Acquired Immune Deficiency Syndrome) is the disease caused by the virus called HIV (human immunodeficiency virus).HIV attacks cells of the immune system and destroys their ability to fight infection by other agents.HIV is spread through the direct exchange of body fluids.There is a long period of time from HIV infection to the onset of AIDS.Anti-HIV drugs prolong the length and quality of life, but there is no vaccine or cure for AIDS.

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Viral Diseases

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Fungi

Fungi are like plants made up of many cells. They are not called plants because they cannot produce their own food from soil and water. Instead, they live off animals, including people, and plants. Mushrooms and yeast are fungi.

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Protozoa

Protozoa are very small. Most live in water. They are parasites, which means they live off other organisms. Malaria and Giardia are parasitic protozoa.

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2- Host

• Hosts are organisms, usually humans or animals, which are exposed to disease.

• The host can be the organism that gets sick, as well as any animal carrier (including insects and worms) that may or may not get sick.

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Types of Host Factors

• Physiological • Anatomical• Genetic • Behavioral• Occupational• Constitutional• Cultural

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Routes that microbes use to exit host

• Respiratory tract: via cough/sneeze

• Gastrointestinal tract GI tract: via feces

• Urogenital: via vaginal/penile secretions

• Skin: via open wounds

• Blood: open wounds, surgery, syringes

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3-Environment, or those external factors that cause or allow disease transmission.

• Favorable surroundings and conditions external to the host that cause or allow the disease to be transmitted.

• Some diseases live best in dirty water. Others survive in human blood. Still others, like E. coli, thrive in warm temperatures but are killed by high heat.

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Types of Environmental Factors

• Physical, chemical, biological• Social, political, economic• Population density• Cultural• factors that affect presence and levels of

agents

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3-Multi-factorial causation

• Is known as modern disease of civilization such as; lung cancer, coronary heart disease, mental illness etc

• these disease is not explained on the basis of the germ theory and could not be prevented by;

• isolation,• immunization • improvement in sanitation

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4-Web of causation

• The web of causation is primarily a medical

term used to represent the complex group of

subjects and relationships that can contribute

to the occurrence and spread of a disease.

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web of causation, an interrelationship of multiple factors that contribute to the occurrence of a disease

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Natural history of diseases• Natural history of disease refers to the

progress of a disease process in an individual over time, in the absence of intervention.

• The process begins with exposure to or accumulation of factors capable of causing disease.

• Without medical intervention, the process ends with recovery ,disability, or death

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Natural history of diseases

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• Pre pathogenicity • Pre-pathogenesis is before the beginning or

onset of a disease

• pathogenicity• the origin and development of the disease, and

whether it is acute, chronic, or recurrent.

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• acute disease; a disease characterized by a relatively sudden onset of symptoms that are usually severe, Examples are pneumonia and appendicitis

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• Chronic disease: A disease that persists for a long time. A chronic disease is one lasting 3 months or more, by the definition of the U.S. National Center for Health Statistics. Chronic diseases generally cannot be prevented by vaccines or cured by medication, nor do they just disappear. Example such as cardiovascular disease such as heart attacks and stroke, cancer such as breast and colon cancer, diabetes

• Recurrent disease; Occurring or appearing again or repeatedly.

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• Types of pathogenesis include;• microbial infection, • inflammation, • malignancy and tissue breakdown.• For example bacterial pathogenesis is the

mechanism by which bacteria cause infectious illness

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Predisposing Factors of Disease

• Makes the body more susceptible to disease or alters the course of the disease

• gender• age• fatigue• climate• poor nutrition

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Progression of disease

• An abnormal condition of a part, organ, or

system of an organism resulting from various

causes, such as infection, inflammation,

environmental factors, or genetic defect,

• and characterized by an identifiable group of

signs, symptoms, or both.

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Progression of disease in an individual over time

Dr. Siham GritlyFour common stages that most disease manifests

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Four common stages that most disease manifests

• Stage of susceptibility ;

• susceptible due to the presence of risk factors.

Individuals with high serum cholesterol,

• hypertension, a sedentary lifestyle, and

diabetes, for example, have an increased risk

of developing coronary heart disease.

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• Stage of pre-symptomatic disease; or stage of subclinical disease

• the disease process has begun, but no overt signs or symptoms are evident to the host.

• For non-communicable diseases, this stage includes the incubation period, (the time between the invasion of an infectious agent and the development of the first signs or symptoms of the disease)

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• Stage of clinical disease

• a stage in the history of a pathologic condition

that begins with anatomic or physiologic

changes that are sufficient to produce

recognizable signs and symptoms of a disease.•

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• Stage of diminished capacity or Stage of recovery, disability or death

• A reduced capacity, caused by mental illness, intoxication, or some other cause of diseases, that prevented a person from normal activities

• Or recovery from disease

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infectious diseases

• Infectious diseases are disorders caused by organisms — such as bacteria, viruses, fungi or parasites.

• infectious diseases result from the interaction of agent, host, and environment

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infectious diseases

• Some infectious diseases can be passed from person to person.

• Some are transmitted by bites from insects or animals.

• And others are acquired by ingesting contaminated food or water or being exposed to organisms in the environment.

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infectious diseases

• Signs and symptoms vary depending on the organism causing the infection, but often include fever and fatigue.

• Mild complaints may respond to rest and home remedies, while some life-threatening infections may require hospitalization.

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infectious diseases

• Many infectious diseases, such as measles and

chickenpox, can be prevented by vaccines.

• Frequent and thorough hand-washing also

helps protect you from infectious diseases.

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Severity of Infectious Disease

• Infectivity refers to the proportion of exposed persons who become infected.

• Measures of Infectivity,• Infectivity (ability to infect)• (number infected / number susceptible) x

100

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• Pathogenicity refers to the proportion of infected persons who develop clinical disease.

• Pathogenicity (ability to cause disease)• (number with clinical disease / number

infected) x 100

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• Virulence refers to the proportion of persons with clinical disease who become severely ill or die

• Virulence (ability to cause death) • (number of deaths / number with disease) x

100• All are dependent on host factors

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Classifying Infectious Diseases

• Communicable Diseases: transmitted directly or indirectly from one host to another

• Contagious Diseases: easily spread from one person to another

• Non-communicable Diseases: not spread from host to another

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Stages of infectious diseasesstages of infectious diseases

Definition

incubation time between infection, signs and symptoms

prodromal mild symptoms

invasive normal signs and symptoms

Acme signs and symptoms at their strongest

decline death and defeat

convalescence healing

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Classification of DiseaseBased on frequency of occurrence:

• Sporadic; appearing or happening at irregular intervals in time; occasional:

• Endemic; Prevalent in or limited to a particular locality, region, or people

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Classification of DiseaseBased on frequency of occurrence:

• Epidemic; a widespread occurrence of an infectious disease in a community at a particular time.

• Pandemic; prevalent over a whole country or the world.

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CHAIN OF INFECTION

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chain of infection

• infectious diseases result from the interaction of agent, host, and environment.

• transmission occurs when the agent leaves its reservoir or host through a portal of exit, and is carried by some mode of transmission, and enters through an appropriate entry to infect a susceptible host.

• This is sometimes called the chain of infection

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6 links in the chain of infection

• The Infectious Agent; -any disease-causing microorganism (pathogen)

• The Reservoir Host; -the organism in which the infectious microbes reside

• The Portal of Exit; route of escape of the pathogen from the reservoir

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• The Route of Transmission; method by which the pathogen gets from the reservoir to the new host

• The Portal of Entry; -route through which the pathogen enters its new host

• The Susceptible Host; -the organism that accepts the pathogen

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references

• Park's Textbook of Preventive and Social Medicine 21 Edition, by K. PARK

• World Health Organization reports