principles of disease and epidemiology. general principles related to disease pathology is the...

Principles of Disease and Principles of Disease and EpidemiologyEpidemiology

General Principles Related to General Principles Related to

DiseaseDisease PathologyPathology is the scientific study of is the scientific study of

disease and it involves three things;disease and it involves three things; The study of the cause orThe study of the cause or etiology etiology of a of a

disease.disease. The study of the manner in which the The study of the manner in which the

disease developsdisease develops The study of the structural and The study of the structural and

functional changes brought about by the functional changes brought about by the disease and its final effects on the hostdisease and its final effects on the host

General Principles Related to General Principles Related to DiseaseDisease

InfectionInfection refers to the invasion or refers to the invasion or colonization of the body by colonization of the body by pathogenic pathogenic microorganisms.microorganisms.

DiseaseDisease refers to any change from a refers to any change from a normal state of health or an abnormal normal state of health or an abnormal state in which part or all of the body is state in which part or all of the body is not properly adjusted or is not capable not properly adjusted or is not capable of carrying on its normal functions.of carrying on its normal functions.


Although infection and disease are often Although infection and disease are often used interchangeably, infection does not used interchangeably, infection does not necessarilynecessarily indicate disease. For example: indicate disease. For example: Someone may be infected with HIV, but show Someone may be infected with HIV, but show

no symptoms of the disease.no symptoms of the disease. Normal floraNormal flora may colonize or infect specific may colonize or infect specific

areas of the body without causing disease. areas of the body without causing disease. They may, however, cause disease given the They may, however, cause disease given the right circumstances.right circumstances. E. coliE. coli is NF in the stool, but it is the number one is NF in the stool, but it is the number one

cause of urinary tract infections.cause of urinary tract infections.


Viridans group Viridans group StreptococcusStreptococcus is normal flora in the is normal flora in the oropharyngeal area, but can cause subacute bacterial oropharyngeal area, but can cause subacute bacterial endocarditis when it gains entrance into the bloodstreamendocarditis when it gains entrance into the bloodstream

Both of the above organisms are considered to be Both of the above organisms are considered to be opportunistic pathogens.opportunistic pathogens.

In addition to the usual NF, some people are In addition to the usual NF, some people are colonized by microorganisms that are generally colonized by microorganisms that are generally considered to be pathogenic, but which don’t considered to be pathogenic, but which don’t actually cause disease in those individuals. actually cause disease in those individuals. These people are called These people are called carrierscarriers and they can transmit and they can transmit

the pathogenic microorganisms to other people who then the pathogenic microorganisms to other people who then develop the disease normally caused by that develop the disease normally caused by that microorganism.microorganism.


Neisseria meningitidisNeisseria meningitidis Salmonella typhiSalmonella typhi

Causes of disease – some diseases Causes of disease – some diseases have a well understood etiology, have a well understood etiology, others have a partially understood others have a partially understood etiology, and others have an etiology, and others have an undetermined etiology. The main undetermined etiology. The main categories of disease include:categories of disease include: Infectious diseaseInfectious disease – caused by disease – caused by disease

producing microorganismsproducing microorganisms


Nutritional deficiency diseaseNutritional deficiency disease – caused by – caused by the lack of a particular, necessary nutrientthe lack of a particular, necessary nutrient

Congenital diseaseCongenital disease** – is present at birth – is present at birth and is the result of some condition that and is the result of some condition that occurred in utero (maternal infection, use occurred in utero (maternal infection, use of drugs or alcohol, etc.)of drugs or alcohol, etc.)

Inherited diseaseInherited disease** – are passed to the – are passed to the child via the parent’s reproductive cellschild via the parent’s reproductive cells * birth defects may result from * birth defects may result from

thesethese


Metabolic diseasesMetabolic diseases – result from – result from abnormalities in the biochemistry of body abnormalities in the biochemistry of body function. function. Many are congenital or inherited disorders.Many are congenital or inherited disorders.

Degenerative diseasesDegenerative diseases – this occurs when – this occurs when there is a wearing down of part of the there is a wearing down of part of the body leading to loss of function. body leading to loss of function. This may be due to aging, excessive caloric This may be due to aging, excessive caloric

intake, radiation, errors in gene function, etc.intake, radiation, errors in gene function, etc.


Neoplastic diseasesNeoplastic diseases – these are tumors which are – these are tumors which are new growth of cells or tissues. new growth of cells or tissues. Tumors may be benign or malignant. What is the Tumors may be benign or malignant. What is the

difference?difference?

Immunologic diseasesImmunologic diseases – this occurs when some of – this occurs when some of our immunologic defenses attack our own bodies. our immunologic defenses attack our own bodies. These diseases are also called These diseases are also called autoimmune diseasesautoimmune diseases..

Iatrogenic diseaseIatrogenic disease – are caused by health care – are caused by health care personnel during the delivery of health carepersonnel during the delivery of health care Could be due to use of contaminated equipmentCould be due to use of contaminated equipment Could be caused by the administration of drugsCould be caused by the administration of drugs


Nosocomial diseaseNosocomial disease – is a hospital – is a hospital acquired diseaseacquired disease Hospitals provide the perfect environment Hospitals provide the perfect environment

for microorganisms, in part because patients for microorganisms, in part because patients are in a weakened condition and therefore are in a weakened condition and therefore more susceptible to disease.more susceptible to disease.

Up to 15% of hospitalized patients get a Up to 15% of hospitalized patients get a nosocomial infection.nosocomial infection.

Most nosocomial infections are caused by Most nosocomial infections are caused by opportunistic pathogens opportunistic pathogens that are typically that are typically considered to be NF.considered to be NF.

Factors contributing to Factors contributing to nosocomial infectionsnosocomial infections

Types of nosocomial Types of nosocomial infectionsinfections

(Septicemia)


Psychogenic diseasesPsychogenic diseases – are caused, at – are caused, at least in part, by emotional factorsleast in part, by emotional factors

Idiopathic diseasesIdiopathic diseases – diseases that have – diseases that have an undetermined causean undetermined cause

Koch’s postulatesKoch’s postulates – in the 1800’s – in the 1800’s Robert Koch developed a series of Robert Koch developed a series of steps, called Koch’s postulates, that steps, called Koch’s postulates, that provide a framework for the study of provide a framework for the study of the etiology of an infectious disease:the etiology of an infectious disease:


The same pathogen must be present in The same pathogen must be present in every every casecase of the disease. of the disease.

The pathogen must be isolated from the The pathogen must be isolated from the diseased host and grown in diseased host and grown in pure culturepure culture..

The pathogen from pure culture must cause The pathogen from pure culture must cause the the same diseasesame disease when inoculated into a when inoculated into a healthy, susceptible host.healthy, susceptible host.

The pathogen must again be isolated from the The pathogen must again be isolated from the inoculated animal and must be shown to be the inoculated animal and must be shown to be the samesame as the originally isolated organism. as the originally isolated organism.

Application of Koch’s postulatesApplication of Koch’s postulates


Why is not not possible to identify the Why is not not possible to identify the etiology of all infectious diseases etiology of all infectious diseases following Koch’s postulates?following Koch’s postulates? Some pathogens can’t be isolated on Some pathogens can’t be isolated on

artificial culture media. Examples?artificial culture media. Examples? Some pathogens cause more than one kind Some pathogens cause more than one kind

of disease. Examples?of disease. Examples? Some diseases are cause by a variety of Some diseases are cause by a variety of

different pathogens. Examples?different pathogens. Examples?


How do infections or diseases spread?:How do infections or diseases spread?: ReservoirsReservoirs – for a disease to perpetuate itself, there – for a disease to perpetuate itself, there

must be a continual source of the infection called a must be a continual source of the infection called a reservoir of infection. reservoir of infection. Reservoirs my be:Reservoirs my be: Humans – Carriers or sick individuals during the incubation, Humans – Carriers or sick individuals during the incubation,

acute, or convalescent phases of a disease (depends upon acute, or convalescent phases of a disease (depends upon the particular disease)the particular disease)

Animals – diseases found in animals are called Animals – diseases found in animals are called zoonosiszoonosis Non-living entities such as soil or waterNon-living entities such as soil or water

Transmission of diseaseTransmission of disease – The causative agents of – The causative agents of infection can be transmitted from the reservoir of infection can be transmitted from the reservoir of infection to a susceptible host via four different infection to a susceptible host via four different routes:routes:


ContactContact Direct contactDirect contact –the infection is spread from person to –the infection is spread from person to

person via touching, kissing, or sexual intercourseperson via touching, kissing, or sexual intercourse Indirect contactIndirect contact – the disease is transmitted to a – the disease is transmitted to a

susceptible host by means of a non-living object susceptible host by means of a non-living object called a called a fomite fomite (contaminated needles)(contaminated needles)

Droplet transmissionDroplet transmission – agents of disease are spread – agents of disease are spread by droplets contained in saliva and mucous that are by droplets contained in saliva and mucous that are discharged by sneezing, coughing, laughing, or discharged by sneezing, coughing, laughing, or talking.talking.

The droplets travel only a short distance (< 1 The droplets travel only a short distance (< 1 meter) and for such short distances, the spread is meter) and for such short distances, the spread is not considered to be airborne.not considered to be airborne.

Direct contact transmissionDirect contact transmission

Droplet transmissionDroplet transmission


Common vehicle transmissionCommon vehicle transmission – refers to – refers to transmission of disease causing agents by a transmission of disease causing agents by a common inanimate reservoir to a large common inanimate reservoir to a large number of individualsnumber of individuals

FoodFood WaterWater BloodBlood DrugsDrugs

Common vehicle Common vehicle transmissiontransmission


Airborne transmissionAirborne transmission – this refers to the – this refers to the spread of agents of infection by droplet spread of agents of infection by droplet nuclei or dust. nuclei or dust.

In this instance the particles travel > 1 meter In this instance the particles travel > 1 meter from the reservoir to the host.from the reservoir to the host.

Vector transmissionVector transmission – a vector is an animal – a vector is an animal that carries the pathogen from one host to that carries the pathogen from one host to another.another.

Arthropods are the most common type of vector.Arthropods are the most common type of vector. Arthropods can transmit disease by two general Arthropods can transmit disease by two general

methods:methods:


MechanicalMechanical – the pathogen is transmitted on – the pathogen is transmitted on the insects feet or other body parts to the the insects feet or other body parts to the hosts food.hosts food.

BiologicalBiological – the arthropod bites an infected – the arthropod bites an infected individual and acquires the pathogen which individual and acquires the pathogen which reproducesreproduces inside the insect. inside the insect.

The pathogen is then transmitted to a new host The pathogen is then transmitted to a new host via the salivary glands or in the feces when the via the salivary glands or in the feces when the insect defecates.insect defecates.

Vector transmissionVector transmission


Classification of infectious diseases – there are Classification of infectious diseases – there are several different ways to classify diseases:several different ways to classify diseases: Based on how the disease is spreadBased on how the disease is spread

Communicable diseaseCommunicable disease – spreads from one host to another – spreads from one host to another either directly or indirectlyeither directly or indirectly

Noncommunicable diseaseNoncommunicable disease – doesn’t spread from host to – doesn’t spread from host to host but is caused by an organism that host but is caused by an organism that

Normally inhabits the body and only occasionally causes Normally inhabits the body and only occasionally causes disease ordisease or

Resides outside the body and produces disease when it is Resides outside the body and produces disease when it is introduced into the bodyintroduced into the body

Contagious diseaseContagious disease – a disease that spreads – a disease that spreads easilyeasily from from one person to anotherone person to another


Diseases may be classified in terms of severity Diseases may be classified in terms of severity or durationor duration Acute diseaseAcute disease – develops rapidly and lasts a relatively – develops rapidly and lasts a relatively

short timeshort time Chronic diseaseChronic disease – develops slowly, the body’s – develops slowly, the body’s

reactions are usually less severe, but the disease is reactions are usually less severe, but the disease is continuous or recurrent for long periods of timecontinuous or recurrent for long periods of time

Subacute diseaseSubacute disease – is intermediate between acute – is intermediate between acute and chronicand chronic

Latent diseaseLatent disease – the causative agent remains inactive – the causative agent remains inactive for a long period of time and then becomes active for a long period of time and then becomes active and produces symptomsand produces symptoms


Diseases can be classified based on the Diseases can be classified based on the frequency of occurrencefrequency of occurrence SporadicSporadic – occurs only occasionally– occurs only occasionally EndemicEndemic – constantly present in the – constantly present in the

populationpopulation EpidemicEpidemic – many people in a given area – many people in a given area

acquire the disease in a short period of timeacquire the disease in a short period of time PandemicPandemic – a world-wide epidemic – a world-wide epidemic


Diseases may be classified based on the Diseases may be classified based on the health of the bodyhealth of the body Primary infectionPrimary infection – an acute infection that – an acute infection that

causes the initial illnesscauses the initial illness Secondary infectionSecondary infection – is caused by an – is caused by an

opportunistic pathogen after the primary opportunistic pathogen after the primary infection has weakened the body’s defensesinfection has weakened the body’s defenses

Inapparent or subclinical infectionInapparent or subclinical infection – doesn’t – doesn’t cause any notable illness (a problem in cause any notable illness (a problem in carriers)carriers)


What are the signals of disease?What are the signals of disease? Symptoms Symptoms – – subjectivesubjective changes in body changes in body

function such as pain or malaise (not function such as pain or malaise (not measurable)measurable)

SignsSigns – – objectiveobjective changes that can be changes that can be observed and measured such as fever, observed and measured such as fever, swelling, or a rashswelling, or a rash

SyndromeSyndrome – a group of symptoms and – a group of symptoms and signs that always accompany a signs that always accompany a particular diseaseparticular disease

General Principles Related to General Principles Related to DiseaseDisease What are the stages of disease?What are the stages of disease?

Period of incubationPeriod of incubation – the time been acquiring – the time been acquiring the infection and the appearance of he first the infection and the appearance of he first sins or symptoms.sins or symptoms. This may be a constant time for every individual who This may be a constant time for every individual who

acquires the infection or a variable time (depends acquires the infection or a variable time (depends upon the disease).upon the disease).

Prodromal periodProdromal period – when the first signs and – when the first signs and symptoms appear.symptoms appear.

Period of illnessPeriod of illness – when the disease is most – when the disease is most acute and the overt signs and symptoms of the acute and the overt signs and symptoms of the disease occur (an increase or decrease in disease occur (an increase or decrease in WBCs may occur here).WBCs may occur here).


Period of declinePeriod of decline – this is where the – this is where the signs and symptoms subside. signs and symptoms subside. If the decline occurs quickly, it is said to If the decline occurs quickly, it is said to

occur by occur by crisiscrisis. . If the decline occurs over a longer period of If the decline occurs over a longer period of

time, it is said to occur by time, it is said to occur by lysislysis..

Period of convalescsncePeriod of convalescsnce - this is where - this is where the person regains strength and the the person regains strength and the body returns to its pre-diseased state.body returns to its pre-diseased state.

Stages of diseaseStages of disease


EpidemiologyEpidemiology – is the study of when and – is the study of when and where diseases occur and how they are where diseases occur and how they are transmitted in a population. transmitted in a population. It is also concerned with various methods of It is also concerned with various methods of

controlling a disease such as use of drugs, controlling a disease such as use of drugs, vaccines, and control of human, animal and vaccines, and control of human, animal and nonliving reservoirs.nonliving reservoirs.

Epidemiology is a major concern of state and Epidemiology is a major concern of state and federal federal public health departmentspublic health departments..

The Centers for Disease Control and Prevention, The Centers for Disease Control and Prevention, CDCCDC, in Atlanta, Georgia, is a central source of , in Atlanta, Georgia, is a central source of epidemiological information in the United States.epidemiological information in the United States.

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