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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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Lesson 6 principles of disease and epidemiology

Lesson 6:Principles of Disease and Epidemiology

June 30, 2014


  • 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 within the host and within a given population

  • 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 within the host and within a given population

  • 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 the Prevalence?

  • 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 the Prevalence?

  • 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 Prevalence?

  • 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) the Prevalence?

  • 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 infections

  • 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

Overview infections

  • 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

  • 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: RESISTANCE OF THE HOST TO THE GROWTH/SURVIVAL OF THE PATHOGEN!!!!

    • 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 occupations, pre-existing illness, and emotional distress all are considered to be predisposing factors

  • 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 occupations, pre-existing illness, and emotional distress all are considered to be predisposing factors

    • 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 occupations, pre-existing illness, and emotional distress all are considered to be predisposing factors 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 occupations, pre-existing illness, and emotional distress all are considered to be predisposing factors

    • 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 occupations, pre-existing illness, and emotional distress all are considered to be predisposing factors

    • 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 occupations, pre-existing illness, and emotional distress all are considered to be predisposing factors

    • 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

Stages of disease
Stages of Disease Illness stage

Spread of infection
Spread of Infection Illness stage

  • 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— Illness stagethe 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 Illness stage

    • 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 Illness stage

    • 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 Illness stage

  • 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)

Exceptions to well cooked food
Exceptions to well-cooked food Illness stage


Exceptions to well cooked food1
Exceptions to well-cooked food Illness stage


Disease transmission
Disease Transmission Illness stage

  • 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

  • Contact Illness stage

    • 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 Illness stage

    • 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 Illness stage are animals that carry pathogens from one host to another

  • Two ways that vectors can transmit pathogens

    • Mechanical transmission

    • Biological transmission

  • Biological transmission Illness stage—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