lesson 6 principles of disease and epidemiology
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Lesson 6: Principles of Disease and Epidemiology. June 30, 2014. Pathology. Pathology - is the scientific study of disease Pathogen —any microbe that can cause a disease Divided into three different parts:

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  • Pathology- is the scientific study of disease
    • Pathogen—any microbe that can cause a disease
  • Divided into three different parts:
    • Etiology-investigates the causes or the origins of disease (what causes the disease?)
    • Pathogenesis-studies the manner in which disease develops (how does pathogen grow and survive in the body?)
    • Pathophysiology-Affects of the disease on the human body (How does the body react to the pathogen?)
infection vs disease
Infection vs. Disease
  • Infection- is the invasion or colonization of the body by microorganisms
  • Disease-occurs when an infection results in any change in an animals state of health
  • Do all infections result in disease????
    • No!!!! Poliovirus may infect humans but signs of “disease” may not be present
normal m icrobiota
Normal Microbiota
  • In normal circumstances, animals are free from microorganisms in utero.
  • Most Microbes do not infect animals until birth
    • Exceptions are T.O.R.C.H. (Taxoplasma, others, rubella, cytomegalovirus, herpes)
    • One of the first microbes encountered during birth are lactobacillus spp.
      • Lactobacilli become the predominant organisms in the gastrointestinal tract
microbes in action lactose intolerance
Microbes in Action:Lactose Intolerance
  • Lactose intolerance comes from the inability to break down the dissacharide lactose (glucose and galactose)
    • Insufficient levels of the enzyme lactase
  • Lactobacillus spp.
    • Gram positive rod shaped bacteria
    • In humans, are found in the vagina and the gastrointestinal tract of humans
    • Converts the dissacharide lactose to lactic acid
      • Prevents the production of copious amounts of gas (carbon dioxide and methane)
      • Unabsorbed sugars and fermentation products also raise the osmotic pressure of the colon, resulting in an increased flow of water into the bowels
normal m icrobiota1
Normal Microbiota
  • In normal circumstances, animals are free from microorganisms in utero.
  • Microbes begin to colonize animals during the birthing process
    • One of the first microbes encountered during birth are lactobacillus spp.
      • Lactobacilli become the predominant organisms in the gastrointestinal tract
  • Other microbes are introduced into the body from the environment through breathing and by feeding
    • Table 4.1 (page 404) lists common microbiota by body regions
normal microbiota
Normal Microbiota
  • Human body contains more bacteria than human cells
    • Typical body contains 1013 body cells but has 1014 bacterial cells (ten times more bacteria than cells)
  • The Human Microbiome Project set forth to analyze the microbiomes of the human body to determine the relationship b/w changes in human microbiome and human health and disease
normal microbiota1
Normal Microbiota
  • The microbiota of the body is thought to be of two varieties:
    • Normal microbiota/flora—microbes that reside permanently within our bodies
    • Transient microbiota—microbes that reside for only a short period of time within the body and disappear (cleared)
  • Neither produce disease under normal conditions
normal microbiota2
Normal Microbiota
  • The location of normal flora in the body depends on several factors
    • Temperature
    • pH
    • Oxygen availability
    • Salinity
    • Sunlight
normal microbiota3
Normal Microbiota
  • Certain factors affect the distribution and composition of a humans natural microbiota
    • Age
    • Diet
    • Health status
    • Personal hygiene
    • Stress
microbiota host interactions
Microbiota/Host Interactions
  • Normal flora can benefit the host by preventing the growth of harmful bacteria (microbial antagonism/competitive exclusion)
    • Competition for nutrients
    • Producing substances that inhibit pathogen growth
    • Changing the environment (pH and oxygen levels)
microbes in action yeast infection
Microbes in Action:Yeast Infection
  • The normal microbiota of a woman’s vagina maintains an acidic pH (~4)
  • Acidic pH inhibits the growth of the yeast Candida albicans
  • Disruption of the normal flora causes a change in pH (pH of vagina becomes neutral)
    • Antibiotics, excessive douching, use of deodorants
  • The neutral pH allows the C. albicans to flourish causing a “yeast infection”
  • The relationship between the normal flora and the host is called symbiosis
    • Commensalism—symbiotic relationship where only one of the organisms benefits and the other is unaffected
      • Staphylococcus epidermidisuses nutrients found in secretions
    • Mutualism—symbiotic relationship that benefits both organisms
      • E. coli uses nutrients but provide vitamins K and B
    • Parasitism—one organism benefits by deriving nutrients at the expense of another organism
      • F. tularenesisuses up the host cells amino acids and lipids
opportunistic pathogens
Opportunistic Pathogens
  • Opportunistic pathogens—organisms that ordinarily do not cause disease but in a particular environment may cause disease
    • Location can cause a bacteria of the normal flora to become pathogenic
      • E. coli harmlessly resides in the large intestine but if it infects the urinary bladder it can cause disease
    • Lowered host resistance
      • AIDS patients become more susceptible to pneumonia (Pneumocystis spp.)
the affect of disease on the body
The Affect of Disease on the Body
  • Each disease results in altered body structure and function and is indicated by several kinds of evidence
    • Symptoms—changes in body function experienced by the patient
    • Signs—objective changes in the body that the physician can observe and measure
    • Syndrome—a specific group of symptoms/signs that always accompany a particular disease

Diseases are often classified in terms of how they behave within the host and within a given population

    • Communicable Disease—diseases that are spread from one host to another
      • Diseases that spread EASILY are said to be contagious
    • Noncommunicable Disease—disease that is not spread from host to host
classifying infectious diseases
Classifying Infectious Diseases
  • The classification of diseases are based on several criteria
    • Occurrence of a Disease
    • Severity or Duration of a Disease
    • Extent of Host Involvement
disease occurrence
Disease Occurrence
  • Disease incidence is the number of people with a disease during a particular time period
    • Indicator of how well the disease spreads
    • Incidence of AIDS in the US in 2007 was 56,300
  • Disease prevalence is the number of people in a population who develop a disease at a specified time, regardless of when it appeared
    • “how many cases altogether”
    • Prevalence of AIDS in the US in 2007 was 1,185,000

How frequently and where a disease occurs also help to classify the disease

    • Sporadic disease occurs occasionally
    • Endemic diseases that are specific to a certain area/location (i.e. African-sleeping sickness)
    • Epidemic are diseases that affect many people in a given area in a relatively short period of time
    • Pandemic refers to an epidemic disease that occurs worldwide
severity or duration of disease
Severity or Duration of Disease
  • Another way of classifying a disease is based on how severe the disease is or how long the disease persists within the body
    • Acute disease is one that develops rapidly but clears quickly
    • Chronic diseases are diseases that develop more slowly but persists or recurs over long periods of time
    • Latent disease is one where the pathogen remains inactive for a long time but can become active to produce symptoms of the disease
location of infection
Location of Infection
  • Infections can also be classified based on the extent or location that the host’s body is affected
    • Local infection is one where the invading microorganisms are restricted to a small area of the body(boils or abscesses)
    • Systemic infection—microorganisms and their products are spread throughout the body (measles or chicken pox)
    • Focal infection—local infection that enters the lymphatic vessels and spreads to other areas of the body (infection of lymph nodes: lymphadenitis)
extent of host involvement
Extent of Host Involvement
  • The body’s response (immune system) to microbes can also affect the severity of the disease
  • Septicemia/Sepsis (blood poisoning) is a systemic infection arising from the multiplication of microbes in the blood
    • Bacteremia—presence of bacteria in the blood
    • Viremia—presence of viruses in the blood
    • Toxemia—presence of toxins in the blood
systemic inflammation response syndrome sirs
Systemic Inflammation Response Syndrome (SIRS)
  • Defined as having 2 or more of the following
    • Fever of 100.4 or less than 96.8 (Farenheit)
    • Heart rate >90 beats per minute (bpm)
    • Respiratory rate >20 breaths per minute
    • Abnormal WBC count (>12,000/µL or <4,000/µL)
  • SIRS can lead to MODS (multiple organ dysfunction syndrome)
    • Process is not known but often debated

The state of host resistance also determines the extent of infections

    • Primary infections—is an acute infection that causes the illness
    • Secondary infections—is an infection that is caused by an opportunistic pathogen after the primary infection has weakened the immune system
    • Subclinical infections—is an infection that does not cause a noticeable illness
herd immunity
Herd Immunity
  • The rate in which a disease spreads depends on the immunity of the population
  • People that are immuned (natural or unnatural) act as a barrier for certain diseases
    • This barrier helps to reduce the occurrence of the disease
  • Herd immunity is when many people of a particular community is immune to a disease therefore reducing the spread of that disease
  • When a pathogen enters into our body there is a series of events that occur leading up to disease development
    • Reservoir for the infectious pathogens
    • Transmission of pathogen to host
      • Direct transmission
      • Indirect transmission
    • Invasion of pathogen into the host cells
    • Pathogenesis (pathogens growth/survival within the host cells)


predisposing factors to disease
Predisposing Factors to Disease
  • Several factors other than the pathogen and the host’s resistance towards the invasion/growth/survival of the pathogen dictate the occurrence of disease
  • Predisposing factors—makes the body more susceptible to a disease’s progression

Some predisposing factors for disease include:

    • Gender
      • Women contract UTIs more frequently then men
      • Men have higher rates of pneumonia contraction
    • Genetics
      • Sickle cell anemia
    • Climate/Weather
      • Respiratory diseases increase during the winter months
    • Age
      • Parkinson’s disease more prevalent in people over 50

Inadequate nutrition, fatigue, environment, habits, occupations, pre-existing illness, and emotional distress all are considered to be predisposing factors

  • Usually difficult to link the importance of one of these factors to a particular disease
    • Lactose intolerance is linked to several disposing factors
development of disease
Development of Disease
  • After a pathogenic infection there is a systematic series of steps that follows
    • These steps are referred to as the development of disease

Incubation period

    • Defined as the interval of time between the initial infection and the development of signs/symptoms of that disease
    • Incubation period can vary depending several factors
      • Infecting microorganisms
      • Virulence (how pathogenic the pathogen is)
      • Number of infecting microorganisms
      • Host resistance to the pathogen
    • Table 15.1 page 431 has a listing of different pathogens and the length of their incubation times

Prodromal period

    • Follows the incubation period in some diseases and is characterized by early or mild symptoms of the disease
    • Relatively short period of time

Period of illness

    • Most severe stage of the disease due to exponential growth of the pathogen
    • Characterized by the exhibition of overt signs and signals of a disease
      • Fevers
      • Myalgia (muscle pain)
      • Photophobia (sensitivity to light)
      • Pharyngitis (sore throat)
      • Lymphadenopathy (swelling of the lymph nodes)
    • If a patient’s immune system does not overcome the rapid growth of the pathogen, they die in this stage

Period of Decline

    • In this stage, the host immune system begins the eradication of the pathogen (clearing of pathogen)
    • Signs and symptoms of the disease begin to subside
    • During this stage, the patient is vulnerable to secondary infections!!!!

Period of Convalescence

    • Final stage of disease progression
    • Host defense (immune system) have mostly cleared the infecting pathogen
    • Patient regains strength and the body returns to the pre-diseased state

People can easily spread disease during the Period of Illness stage

    • Pathogen is most abundant.
  • Patients can also spread disease during the Period of Decline and Convalescence stage as well
    • Pathogens are still present. Sometimes the pathogen is never cleared.
spread of infection
Spread of Infection
  • In order to be infected with a pathogen, we first must come into contact with that pathogen
    • Where do these pathogens reside????

Reservoirs of Infection—the place where a pathogen lives

    • This can be a living organism
      • Humans
      • Animals
    • Inanimate objects also serve as reservoirs
      • Swimming pools and gardens

Human Reservoirs

    • Principal living reservoir of human disease
    • Carriers are people who harbor pathogens and can transmit it to others
    • People with no signs/symptoms can also spread disease
  • Why are people that show no signs most dangerous to spread disease?

Animal reservoirs

    • Both wild and domestic animals are living reservoirs of disease
    • Any diseases that normally occur in animals and are capable of infecting humans are called zoonoses
    • Some common diseases from animals are
      • Rabies (bats, dogs, raccoons, squirrels)
      • Lyme disease (mice and other rodents carry ticks)
      • Toxoplasmosis (cats)

Non-living reservoirs

  • Most common is soil and water
    • Soil
      • Soil where animal feces are used as fertilizer are prime reservoirs
      • Fungi
      • Clostridium botulinum
      • Clostridium tetani
    • Water
      • Contaminated water from waste run-offs
      • Vibriocholerae
  • Food can serve as a reservoir (ill-prepared food)
disease transmission
Disease Transmission
  • The passing of the pathogen from the reservoir to a susceptible host is called transmission
  • Pathogens can be transmitted from the reservoir to the host via three routes:
    • Contact
    • Vehicles
    • Vectors


    • Direct contact transmission—transmission of a pathogen by physical contact with carrier
      • “person-to person transmission”
    • Indirect contact transmission—transmission of a pathogen to a host via an inanimate object
      • Fomite—non-living object involved in spread of pathogen (ex. doorknobs, drinking cups, money)
    • Droplet transmission—spread of pathogen via mucous (ex. spit from sneezing, coughing, talking)

Vehicle Transmission

    • Transmission of pathogens through a medium
      • Medium can be water, food, or dust
    • Waterborne transmission—pathogens are usually spread by contaminated water
    • Foodborne transmission—incompletely cooked, poorly stored, prepared under unsanitary conditions
    • Airborne transmission—spread of pathogens through dust. (Staph and Strep spp.survive on dust particles
  • What are some other mediums in which pathogens can be transmitted?

Vectors are animals that carry pathogens from one host to another

  • Two ways that vectors can transmit pathogens
    • Mechanical transmission
    • Biological transmission

Biological transmission—complex process that can be broken down into three parts

    • Vector bites the diseased individual and ingests blood (bloodmeal)
    • The pathogen in the blood begins to grow inside the vectors gut
    • The vector bites another individual and spreads the pathogen
      • Passed via vomit or feces