1. Infection and Disease

2. Pathology, Infection and Disease
• Pathology: scientific study of a disease
• Etiology: cause/ origin of a disease
• Pathogenesis: mechanism by which a disease
develops
Pathologists are concerned with etiology and
pathogenesis of disease and effect of disease on
human body.

3. Pathology, Infection and Disease
• Infection: invasion or colonization of body by
pathogenic bacteria
– Also presence of bacteria where they do not
belong
• E. coli in urinary tract
• Disease: change of state of health resulting
from an infection
– Abnormal state
– Part of whole body is incapable of performing
normal function

4. Normal Microbiota
• Normal Microbiota
– Normal flora
– Permanent residents
– Don’t normally cause disease
• Transient Microbiota
– Present for days/months/years then disappear
Only certain body areas are colonized:
Nasal epithelium Stomach lining Large intestine

5. Normal Microbiota
• Skin
– Staphylococcus, Micrococcus,
Candida (fungus)
– Salt and low pH inhibitory for
others
• Eyes
– Similar to those on skin
– Tears and blinking
• Prevent others from colonization
Staph epidermis SEM

6. Normal Microbiota
• Nose & Throat
– S. aureus, S. epidermidis, Haemophilus
– Mucus & cilia remove/inhibit most microbes
• Mouth
– Streptococcus, Staphylococcus, Lactobacillus
– Warm moist environment, food particles as nutrients
SEM of bacteria on the human
tongue, most of which are
harmless or even beneficial

7. Normal Microbiota
• Large intestine
– E. coli, Lactobaccillus, Enterobacter, Candida (fungus),
Enterococcus
– Largest numbers
• Moisture and nutrients
• Urogenital system
– Lactobacillus, Staphylococcus, Micrococcus,
Enterococcus, Pseudomonas
– Urine flushes out microbes
– Low pH inhibits microbes

8. Factors Affecting Distribution and
Composition of Microbiome
• Nutrients
• Temperature
• pH
• Oxygen
• Salinity
• Sunlight
• Host Defenses
• Individual Criteria
– Age, Diet, Geography, Health, Stress

9. Normal Microbiota: Host Relationship
• Microbial Antagonism/Competitive Exclusion
– Prevent growth of other (pathogenic) microbes
• E. coli bacteriocins kill others (Salmonella/ Shigella)
– Alter environment
• Lactobacillus lower vaginal/ intestinal pH
– Less desirable for other microbes like Candida
– Compete for nutrients
• Growth of C. difficile is kept in check by intestinal
microbiome

10. Normal Microbiota: Host Relationship
• Symbiosis: relationship between two
organisms with at least one dependent on the
other
– Commensalism: one benefitted, other unaffected
• Corynebacteria: NF of eye, no damage/benefit to host
– Mutualism: both benefit
• E. coli in gut: nutrients used; synthesizes Vit. K/ B
vitamins
– Parasitism: one benefits at expense of other
• Pathogenic microbes

11. Normal Microbiota: Host Relationship
• Opportunistic Pathogens
– Normal Flora: don’t cause disease in their specific
niche
– Infection when they access a different environment
– Immune suppression
– E. coli: mutualistic in large intestine
– Pathogen
• UTI
• Wound abscess
• meningitis

12. Koch’s Postulates: Etiology of a Disease
• Robert Koch: 1877
– Experimental Requirements to determine the
microbial cause of a disease
1. The same pathogen must be present in every case of
the disease
2. Pathogen must be isolated from diseased host and
grown in pure culture
3. Pathogen from pure culture must cause disease
when inoculated into a healthy host
4. Pathogen must be isolated from the new host and
shown to be the same as the original organism

13. Koch’s Postulates

14. Koch’s Postulates: Exceptions
• Cannot satisfy Postulate 2
– Some pathogens cannot be cultured on artificial media
• Viruses (and other oligate intracellular parasites)
• Fastidious organisms (mycoplasma)
• Some pathogens cause several different diseases
– M. tuberculosis: lungs, bones, skin, internal organs
– S. pyogenes
• Some diseases caused by different pathogens
– Nephritis (kidney inflammation)
– Pneumonia, meningitis, peritonitis
• Viruses that cause Cancer

15. Disease Classification
• Communicable Disease: spreads from host to host
(directly/ indirectly)
• Contagious Disease: easily spread from host to host
– Influenza
• Non-communicable disease: not spread from one host
to another
– tetanus
Rusty nails are a prime habitat for
Clostridium tetani endospores

16. Disease Occurence
• Incidence: number of people that develop a
disease in a given time period
– Indicates the rate of disease spread
• Prevalence: TOTAL number of cases in a given
population at a given time
– Indicates how long disease affects population
2007: Incidence of AIDS in US: 56,300
Prevalence: 1, 185,000

17. Disease Occurence
• Sporadic: occurs only occasionally in given population
– Typhoid fever
• Endemic: constantly present in population
– Common cold
• Epidemic: acquired by many people in a given area in a
short time period
– Influenza
• Pandemic: worldwide epidemic
– Some influenza epidemics (H1N1)
– SARS
– AIDS

18. Disease Severity/ Duration
• Acute: rapid but short time
– Influenza
• Chronic: slow progression, long duration
– Mononucleosis
– Tuberculosis
– Hepatitis B
• Subacute: intermediate between acute and chronic
– Subacute sclerosing panencephalitis
• Latent: causative agent remains inactive, can reactivate
to produce symptoms
– Herpesvirus

19. Extent of host involvement
• Classify infections based on the affect on host
– Local infection: relatively small area
• Abscess, boils
– Systemic (generalized): spread through the body
by blood/ lymph
• Local infection can also get systemic
• measles
– Focal infection: start in one area but spread to
another confined area via blood/ lumph
• Dental plaque linked to heart disease

20. Extent of host involvement
• Sepsis: whole body inflammation by microbial
spread
• Septicemia: blood poisoning
– Systemic infection from pathogens multiplying in
blood
– Toxemia, Bacteremia, Viremia
• Primary infection: acute infection causing initial
illness
• Secondary infection: opportunistic pathogen
infection because of weakened immune system

21. Steps in disease development
• Incubation Period
– Before signs and symptoms appear
• Prodromal period
– Early, mild, non-specific symptoms appear
• Period of illness
– Disease specific symptoms
– Symptoms most severe
– Immune system overcomes pathogen OR patient dies
• Period of decline
– Symptoms subside
– Vulnerable to secondary infections
• Period of convalescence
– Body returns to pre-diseased state
Patient can be infectious at EVERY stage of disease

22. Patterns of disease
For disease to occur, the following conditions need
to be met:
 Reservoir of infection
 Source of pathogens
 Transmission
 Route from reservoir to host
 Invasion
 Pathogen enters host and multiplies
 Pathogenesis
 Causes damage to host

23. Reservoirs of infection
• Human
– Lacking disease symptoms
• Carriers
• Animal reservoirs
– Zoonoses
• Animal diseases transmitted to humans
• Rabies, Lyme disease
• Non-living reservoir
– Contaminated water
• Vibrio, Salmonella
– Soil
• Clostridium
• Fungi (ringworm)

24. Reservoirs of infection: Carriers
Typhoid fever = infection of Salmonella typhi characterized by
high fever, headache and diarrhea
• Spread only through human feces
Chronic carriers: recovered patients that harbor S. typhi in gall
bladder – may shed bacteria indefinitely
“Typhoid Mary” Mallon
• Early 1900s – worked as a cook in NY
• Linked to several typhoid outbreaks and deaths
• Numerous attempts to restrain her from food industry

25. Transmission of infection: Contact
• Direct
• Indirect (fomites: inanimate objects)
• Droplet (mucus; travel < 1m)

26. Transmission of infection: Vehicles
• Waterborne
– Contaminated with feces
• Foodborne
– Improperly cooked/ stored/ prepared
• Airborne
– Travels > 1m to host
– Fungal spores, TB, measles

27. Transmission of infection: Vectors
• Mechanical
– Flies sit on feces, sit on food
– Pathogens ingested
• Biological
– Insect bites infected host
– Pathogen replicates in insect
– Insect bites uninfected host and transmits
pathogen
– Lyme disease, Dengue, plague, malaria

28. Nosocomial Infections
• Direct Contact
– Staff to patient
– Patient to patient
• Indirect Contact
– Fomites (Catheters, needles)
– Airborne (ventilation system)
Prevention of transmission
• Aseptic Technique/ Sterilization of equipment
• Hand washing
• Proper handling/ disposal of contaminated material

29. Transmission in hospitals
Nosocomial infections in the United States
• Eighth leading cause of death
• Occur because of three factors:
 Presence of pathogens
 Chain of transmission
 Immune compromised host

30. Nosocomial infections: most common sites

31. Invasion and Pathogenesis
• Host v/s pathogen
– Host defenses prevent invasion by pathogen
• No disease
– Pathogen outwits host defenses
• disease

32. Predisposing Factors
• Alter disease susceptibility
• Alter disease course
• Genetics
– Sickle cell disease
• Heterozygous: sickle cell trait
• Homozygous: sickle cell disease
– Deliver less oxygen
– Stuck in capillaries
– Anemia, organ failure
– Highest prevalence in West Africa
– Confers resistance to MALARIA

33. Predisposing Factors
Genetic predisposing factors continued
Malaria = infectious disease caused by the proliferation of
the eukaryotic parasite plasmodium in RBCs
• Vector = mosquito
• plasmodium can’t
multiply in sickled cells
So…
 While sickle cell disease is very unfavorable,
 sickle cell trait provides heightened survival in malaria
prone areas
• ensures retention of sickle cell gene

34. Predisposing Factors
Other predisposing factors:
• Environment (nutrition, occupation, lifestyle, climate)
• Age
• Immunity
• Gender
Above: Bear Grylls is environmentally predisposed to
all sorts of diseases

35. Predisposing Factors
Ex) Climate & weather
• Increased respiratory diseases in winter
• Proximity to others while confined indoors & poor
ventilation
• Breathing cold air  cough, sneeze, runny nose 
transmits pathogens

36. Invasion
• Portals of Entry: mucous membrane
– Conjunctiva: mucous membrane of eyelid and eyeball
– Respiratory tract
• Most frequently used portal
• Moisture and dust inhaled through nose and mouth
• Pneumonia, tuberculosis, influenza
– GI tract
• Contaminated food and water
• Must survive stomach acid, bile in small intestine
• Eliminated in feces: can further contaminate

37. Invasion
• Portal of Entry: Skin
– Impenetrable by microbes
• Unless broken
– Penetrate through hair follicles, sweat ducts
– Some fungi and eukaryotic parasites
• Infect unbroken skin
• Penetrate intact skin (ex. Hookworm)

38. Invasion
• Portal of Entry: Parenteral
– Deposited under skin or mucous membrane if
barrier broken/penetrated
– Cuts, burns, bites, wounds, injections
– Tetanus from puncture wound
– Malaria from mosquito bite
– HepB/ HepC from injection

39. Invasion
• Size Matters!
– Larger the “inoculum” more likely the chance to cause disease
– Harder to overcome host defenses
• ID50
– Infectious dose where 50% population is infected
– Compare relative virulence under experimental condition
• ID50 of Bacillus anthracis through skin is 10-50 endospores, ID50
through inhalation is 10,000-20,000
• LD50
– lethal dose where 50% of population is killed
– Measure of potency of microbial toxin
• botulism toxin LD50 = 0.03 ng/kg; Shiga toxin LD50 = 250 ng/kg

40. Invasion
• Adherence
– Adhesins on microbe to bind to cell receptors
– Primarily glycoproteins and lipoproteins
• Bind host receptors: sugars (mannose)
– Adhesins can bind other bacteria
• Actinomyces bind glycocalyx of S. mutans on teeth
– Biofilms
• Microbes bind to living/non-living
• Dental plaque, algae on swimming pool walls, scum on shower
stalls
• First microbes in biofilm usually bacteria
• Can colonize heart valves, catheters, contact lens
– Difficult to treat with antibiotics

41. Need to avoid host defenses to survive
• Antiphagocytic factors
– Capsules, cell wall proteins (M-proteins)
– coagulase (walls off)
– survival in WBC - intracellular parasites
(mycobacteria, Listeria)
• Opportunistic pathogens need compromised host
• Antigenic Variation
N. meningitidis crosses
blood-brain barrier

42. How do bacteria damage the host?
• Endotoxin
• Exotoxins
– Toxins acting on cell membranes
– Toxins active inside cells
– Superantigens
–Non-specific proliferation
of T-cells  cytokines

43. Exotoxins
– Proteins expressed by bacteria during replication
– Secreted into media or released on lysis
– Made by some gram (+) and gram (-) bacteria
– Water soluble, easily diffuse into lymph/ blood
– Extremely lethal
• Cholera Toxin
– Causes cells to secrete fluids & electrolytes  diarrhea
• Tetanus neurotoxin
– Binds nerve cells, prevents muscle relaxation, causes
convulsive muscle contractions

45. Endotoxins
• Part of bacterial cell wall
– Outer membrane of gram (-)
– Lipid A of LPS
– Released on cell death
– Symptoms for all endotoxins same
• Chills, fever, aches
• Shock (decrease in BP)
– S. typhi – typhoid fever
– Neisseria meningitidis: meningococcal meningitis

46. Exotoxins & Endotoxins Compared

48. Signs and Symptoms
Signs: Objective findings observed by
healthcare worker
Symptoms: patient complains in own words

49. Epidemiological Triad
The traditional model of infectious disease causation. Includes three components: an
external agent, a susceptible host, and an environment that brings the host and agent
together, so that disease occurs.
Science that evaluates the occurrence, determinants, distribution,
and control of health and disease in a defined human population
Epidemiology-

50. Definitions
• Epidemiology
– is the study of the behavior of disease in a community
rather than individual patients. Its incidence and spread.
– It includes the study of the reservoirs and sources of human
diseases.
• Epidemiologists
– looks at the factors involved in the occurrence and spread of
disease within populations of humans or animals.
• AIM:
– To understand the mode of transmission and what
predispose a population to a particular agent

51. Types of studies: Descriptive
1. Retrospective
• Determine the source/ cause of infection
after the fact
• Cholera outbreak in Britain 1848-1849
• SARS
2. Prospective
• Follow a group of healthy people
• Study the effect of subsequent disease
• Salk polio vaccine
• UF Flu study

52. Types of Study:
Analytical Epidemiology
Analyze particular disease to determine cause
1.Cohort
– Two populations
• One who had contact with disease
• Those that had blood transfusions v/s not
– Hep B association
2.Case-control
– Identify factors that preceded disease
• Compare diseased v/s disease-free individuals
• Matched by sex, age, socioeconomic status, location

53. Experimental Epidemiology
• Hypothesis
• Experiments to test hypothesis
• Drug Effectiveness
– Select infected people
– Randomly select those that get drug v/s placebo
– Compare results