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Principles of Disease and Epidemiology. Pathology. Branch of medicine to study disease Cause or etiology of disease Manner which disease develops Structural and functional changes by disease and effects on host. Infection and Disease. Exposure – left unprotected to contact with MO

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Principles of disease and epidemiology

Principles of Disease and Epidemiology


  • Branch of medicine to study disease

  • Cause oretiologyof disease

  • Manner which disease develops

  • Structural and functional changes by disease and effects on host

Infection and disease
Infection and Disease

  • Exposure – left unprotected to contact with MO

  • Infection - invasion or colonization of body by pathogenic MO

  • Disease - any change from normal state of health or abnormal state, in which body not properly adjusted or not capable of carrying on normal function

Infection and disease1
Infection and Disease

  • Infection not necessarily indicate disease

  • Normal flora - may colonize or infect specific areas of body without causing disease; may however cause disease given right circumstances

  • Person infected with MO (i.e., HIV) but show no symptoms of disease

Normal flora opportunist
Normal Flora: Opportunist

  • E. coli NF in stool, but #1 cause of urinary tract infection

  • Viridans group Streptococcus NF in oropharyngeal area, but cause subacute bacterial endocarditis when gains entrance into bloodstream

  • Considered opportunistic pathogen

Normal flora carriers
Normal Flora: Carriers

  • In addition to usual NF, some persons colonized by MO generally considered pathogenic, but without disease

  • These individuals are carriersand transmit pathogenic MO to others who develop disease

  • Neisseria meningitidis – URT

  • Salmonella typhi – GI tract, gall bladder (reservoir site)

Types of infectious disease
Types of Infectious Disease

  • Infectious disease – by disease producing MO

  • Congenital disease – present at birth and result of condition in utero (maternal infection, use of drug or alcohol, etc.); may result in birth defects

  • Iatrogenic disease – caused by health care personnel during delivery of care; could be due to use of contaminated equipment, administration of drug, etc.

  • Nosocomical disease - hospital acquired disease

  • Idiopathic disease – undetermined cause

Nosocomial infections
Nosocomial Infections

  • Hospitals provide perfect environment for MOs, in part because patient with weakened condition and more susceptible to infection and disease

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

  • Most nosocomial infections caused by opportunistic pathogens typically considered NF

Types of nosocomial infections
Types of Nosocomial Infections


Koch s postulates etiology of infectious disease
Koch’s Postulates: Etiology of Infectious Disease

  • 1 – Pathogen present in every case of disease

  • 2 - Pathogen isolated from diseased host and grown in pure culture

  • 3 - Pathogen must cause same disease when inoculated into healthy, susceptible host

  • 4 - Pathogen must again be isolated from inoculated animal, shown to be same as originally isolated MO

  • Not possible to identify etiology of all infectious diseases following Koch’s postulates:

    • MO not isolated on culture media

    • MO not inoculated into healthy human host to cause disease


  • Study of when and where diseases occur and how transmitted in a population

  • Also concerned with methods of controlling and preventing a disease; drugs, vaccines, and reservoirs

  • State and Federal Public Health Department

  • The Centers for Disease Control and Prevention, (CDC), Atlanta, Georgia

  • The World Health Organization (WHO), within the United Nations

Spread of disease reservoirs
Spread of Disease: Reservoirs

  • For a disease to perpetuate, must be a source of infection called a reservoir

  • Humans – Carriers or sick individuals during incubation, acute, or convalescent phases (when transmitted depends on disease)

  • Animals – diseases found in animals called zoonosis

  • Non-living entities - such as soil or water

Spread of disease transmission
Spread of Disease: Transmission

  • Causative agents transmitted from the reservoir of infection to a susceptible host via four main routes:

  • 1. Contact

  • 2. Common Vehicle

  • 3. Airborne

  • 4. Vector

1 contact transmission
1. Contact Transmission

  • Direct contact – spread from person to person via touching, kissing, or sexual intercourse

  • Indirect contact – to a susceptible host by means of non-living object called fomite(i.e., contaminated needles)

  • Droplet transmission – spread by droplets in saliva and mucous; discharged by sneezing, coughing, laughing, or talking. The droplets travel only short distance (< 1 meter) and not considered airborne

2 common vehicle transmission
2. Common Vehicle Transmission

  • Refers to transmission of disease causing agents by a common inanimate reservoir to a large number of individuals

    • Food

    • Water

    • Blood

    • Drugs

3 airborne transmission
3. Airborne Transmission

  • Refers to spread of agents of infection by droplet nuclei or dust

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

  • i.e., air condition unit

4 vector transmission
4. Vector Transmission

  • An animal that carries pathogen from one host to another

  • Arthropods are most common type

  • Transmit disease either by:

    • Mechanical – pathogen on feet or other body parts to food ingested by humans

    • Biological – arthropod bites infected host, acquires the pathogen, MO reproduceinside the vector, pathogen transmitted to new host via salivary glands or feces

Infectious disease how spread
Infectious Disease: How Spread

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

  • Noncommunicable disease – doesn’t spread from host to host but caused by MO:

    • Normally inhabits body and occasionally causes disease

    • Resides outside the body and produces disease when introduced into the body

  • Contagious disease – a disease that spreads easily from one person to another

Infectious disease severity duration
Infectious Disease: Severity/Duration

  • Acute disease – develops rapidly, lasts relatively short time i.e. “cold”

  • Chronic disease – develops slowly, body’s reaction usually less severe, but disease is continuous or recurrent for long period i.e. TB

  • Subacute disease – intermediate between acute and chronic i.e. “infectious” hepatitis

  • Latent disease – MO remains inactive for long period, then becomes active and produce symptoms i.e. chickenpox/shingles

Infectious disease frequency of occurrence
Infectious Disease: Frequency of Occurrence

  • Sporadic– occurs occasionally i.e. measles in USA

  • Endemic – constantly present in population i.e. TB inner city LA

  • Epidemic – many people in a given area acquire disease in short period of time i.e. pertussis in USA 2010

  • Pandemic – worldwide epidemic i.e. flu

Infectious disease health of body
Infectious Disease: Health of Body

  • Primary infection – acute infection causes initial illness

  • Secondary infection – by opportunistic pathogen after primary infection has weakened the body’s defenses

  • Inapparent or subclinical infection – doesn’t cause any noticeable illness (i.e., healthy carriers)

Infectious disease signals
Infectious Disease: Signals

  • Symptoms – subjective changes in body function such as pain or malaise (not measurable)

  • Signs – objective changes that can be observed and measured; such as fever, swelling, or rash

  • Syndrome – a group of symptoms and signs that always accompany a particular disease

Infectious disease stages
Infectious Disease: Stages

  • Period of incubation – time between acquiring infection and appearance of first symptoms/signs

  • Prodromal period – first signs/symptoms appear

  • Period of illness – disease most acute, overt signs/symptoms occur (increase or decrease in WBCs may occur here)

Infectious disease stages1
Infectious Disease: Stages

  • Period of decline – where signs/symptoms subside

    • If the decline occurs quickly, it is said to occur by crisis

    • If the decline occurs over longer period of time, it is said to occur by lysis

  • Period of convalescence - where person regains strength and body returns to pre-diseased state

Class assignment
Class Assignment

  • Textbook Reading:

    • Chapter 2 Host- Pathogen Interaction

      • B. Pathogenesis of Infection

        • Routes of Transmission

  • Key Terms

  • Learning Assessment Questions

  • Review, Review, Review!