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Chapter 14. Principles of Disease and Epidemiology. Principles of Disease and Epidemiology. Pathology Study of disease Disease in a state of not being healthy, change from health Pathogens disease causing organisms Etiology Study of the cause of a disease

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chapter 14

Chapter 14

Principles of Disease and Epidemiology

principles of disease and epidemiology
Principles of Disease and Epidemiology
  • Pathology Study of disease
  • Disease in a state of not being healthy, change from health
  • Pathogens disease causing organisms
  • Etiology Study of the cause of a disease
  • Pathogenesis Development of disease
  • Infection Colonization or invasion of pathogens, may be microbes in the wrong place (E. coli in the urinary tract)
normal microbiota flora and the host
Normal Microbiota (Flora) and the Host
  • Normal Flora - the normal bacteria in you and on you
    • You have 1013 eucaryotic cells and 1014 prokaryotic cells
    • Within 8 - 12 hours of life you are colonized by normal flora (microbiota).
      • Breast feeding versus bottle - different organisms
  • Transient microbiota may be present for days, weeks, or months
  • Microbial antagonism
    • Normal microbiota overwhelm pathogens - no place for them to colonize
      • Intestines and vagina - excessive antibiotics disrupts balance
        • Vagina normally pH ~ 4 with Lactobacillus spp without can lead to Candida infections
  • Symbiosis is the relationship between normal microbiota and the host
normal microbiota and the host
Normal Microbiota and the Host:
  • In commensalism, one organism is benefited and the other is unaffected.
  • In mutualism, both organisms benefit.
    • Be able to give a few examples of mutualistic bacteria
  • In parasitism, one organism is benefited at the expense of the other.
  • Some normal microbiota are opportunistic pathogens.
    • E. coli and urinary tract
    • Pneumocystis carinii and respiratory system
    • Streptococcus pneumoniae and pneumonia
normal microbiota and the host5
Normal Microbiota and the Host:
  • Locations of normal microbiota on and in the human body

Figure 14.2

normal microbiota and the host6
Normal Microbiota and the Host:
  • Microbial antagonism is competition between microbes.
  • Normal microbiota protect the host by:
    • occupying niches that pathogens might occupy
    • producing acids
    • producing bacteriocins
  • Probiotics are live microbes applied to or ingested into the body, intended to exert a beneficial effect.
    • Lactobacillus spp
koch s postulates
Koch’s Postulates
  • Koch's Postulates are used to prove the cause of an infectious disease.

Figure 14.3.1

koch s postulates8
Koch’s Postulates
  • Koch's Postulates are used to prove the cause of an infectious disease.
  • Problems with Koch’s Postulates:
  • Not all diseases have bacterial etiologies
    • Genetic
    • Degenerative
    • Congenital
  • Exceptions
    • Not culturable
      • Treponema / Rickettsia / Chlamydia / viruses
    • Some pathogens cause many different diseases

Figure 14.3.2

classifying infectious diseases
Classifying Infectious Diseases
  • Symptom A change in body function that is felt by a patient as a result of disease
  • Sign A change in a body that can be measured or observed as a result of disease.
  • Syndrome A specific group of signs and symptoms that accompany a disease.
classifying infectious diseases10
Classifying Infectious Diseases

Diseases may be grouped by how spread

  • Communicable disease A disease that is spread from one host to another.
  • Contagious disease A disease that is easily spread from one host to another.
  • Noncommunicable disease A disease that is not transmitted from one host to another. Example: Clostridium tetani
by occurrence of disease
By occurrence of Disease
  • Incidence Fraction of a population that contracts a disease during a specific time.
  • Prevalence Fraction of a population having a specific disease at a given time.
  • Sporadic disease Disease that occurs occasionally in a population.
  • Endemic disease Disease constantly present in a population.
  • Epidemic disease Disease acquired by many hosts in a given area in a short time.
  • Pandemic disease Worldwide epidemic.
  • Herd immunity Immunity in most of a population.
by severity or duration of a disease
By Severity or Duration of a Disease
  • Acute disease Symptoms develop rapidly
  • Chronic disease Disease develops slowly
  • Subacute disease Symptoms between acute and chronic
  • Latent disease Disease with a period of no symptoms when the patient is inactive Shingles
by extent of host involvement
By Extent of Host Involvement
  • Local infection Pathogens limited to a small area of the body
  • Systemic infection An infection throughout the body
  • Focal infection Systemic infection that began as a local infection
  • Bacteremia Bacteria in the blood
  • Septicemia Growth of bacteria in the blood
extent of host involvement
Extent of Host Involvement
  • Toxemia Toxins in the blood
  • Viremia Viruses in the blood
  • Primary infection Acute infection that causes the initial illness
  • Secondary infection Opportunistic infection after a primary (predisposing) infection Pneumocystis pneumonia and AIDS
  • Subclinical disease No noticeable signs or symptoms (inapparent infection) Hepatitis / Typhoid mary / Polio
predisposing factors
Predisposing Factors
  • Make the body more susceptible to disease
    • Short urethra in females
    • Inherited traits such as the sickle-cell gene
    • Climate and weather
    • Fatigue and Stress
    • Age
    • Lifestyle
    • Chemotherapy
    • Gender
development of disease
Development of Disease

Stages of Disease

  • Incubation infection up to first symptoms may or may not be variable
  • Prodromal short period of early mild symptoms -- malaise
  • Period of Illness overt signs -- fever and chills, swollen lymph nodes, GI disturbance increase in WBC’s
  • Period of Decline signs and symptoms subside - susceptible to

2˚ infections

  • Period of Convalescence regain strength and recovery BUT maybe reservoir
reservoirs of infection
Reservoirs of Infection
  • Reservoirs of infection are continual sources of infection.
    • Human — AIDS, gonorrhea
      • Carriers may have inapparent infections or latent diseases.
      • Carriers may be in pre-symptom stage or recovery of a disease - no symptoms
    • Animal — Rabies, Lyme disease
      • Some zoonoses may be transmitted to humans
        • Plague / psittacosis / swine flu / bird flu
    • Nonliving — Botulism, tetanus
      • Soil
      • Water - rivers, lakes, snow, oceans and laundry water
transmission of disease
Transmission of Disease
  • Three main routes
    • Contact - Direct or indirect
    • Vehicles - inanimate objects - e.g. food or drugs
    • Vectors - arthropods


    • Direct Requires close association between infected and susceptible host
    • Indirect Spread by fomites (inanimate objects) glass, toothbrush or clothing
    • Droplet Transmission via airborne droplets
transmission of disease20
Transmission of Disease

Figure 14.6a & 8

transmission of disease21
Transmission of Disease
  • VehicleTransmission by an inanimate reservoir (food, water) Shigella, cholera, airborne on dust aerosol >3’, tapeworm Staphylococci, Streptococci, tuberculosis, fungal spores -- histoplasmosis, coccidiodomycosis
  • Vectors Arthropods, especially fleas, ticks, and mosquitoes
    • Mechanical Arthropod carries pathogen on feet
    • BiologicalPathogen reproduces in vector and bites host. Dengue fever, Yellow fever, Malaria, encephalitis, plague, Lyme disease RMSF
nosocomial hospital acquired infections
Nosocomial (Hospital-Acquired) Infections
  • Are acquired as a result of a hospital stay
  • 5-15% of all hospital patients acquire nosocomial infections

Figure 14.7, 9


Hospital acquired

  • 5 - 15% acquire >20,000 per year die
  • why?
  • a) microbes in environment -- (lots of sick people)
  • b) already sick or wounded -- compromised host
  • c) close to people - chain of transmission
  • Also resistant strains - E. coli, Pseudomonas, enterics like Serratia
  • Control by aseptic techniques
  • Antibiotic abuse
emerging infectious diseases
Emerging Infectious Diseases
  • Diseases that are new, increasing in incidence, or showing a potential to increase in the near future.
  • Contributing factors:
    • Evolution of new strains
      • V. cholerae O139
    • Inappropriate use of antibiotics and pesticides
      • Antibiotic resistant strains
    • Changes in weather patterns
      • Hantavirus
    • Spread of human populations and travel
emerging infectious diseases28
Emerging Infectious Diseases
  • Contributing factors:
    • Modern transportation
      • West Nile virus
    • Ecological disaster, war, expanding human settlement
      • Coccidioidomycosis
    • Animal control measures
      • Lyme disease
    • Public Health failure
      • Diphtheria
  • The study of where and when diseases occur

Figure 14.11

centers for disease control and prevention cdc
Centers for Disease Control and Prevention (CDC)
  • Collects and analyzes epidemiological information in the U.S.
  • Publishes Morbidity and Mortality Weekly Report (MMWR)

Morbidity: incidence of a specific notifiable disease

Mortality: deaths from notifiable diseases

Morbidity rate = number of people affected/total population in a given time period

Mortality rate - number of deaths from a disease/total population in a given time