2. Pathology: the study of disease
Etiology: focuses on the cause of a disease
Pathogenesis: the development of a disease
Infection: invasion of the body by pathogens
Disease (morbidity): any change from a state of health
Pathogenicity: a microorganism’s ability to cause disease
Virulence: a measure (degree) of pathogenicity
Terminology
3. Symbiosis indicates the relationship
between the normal microbiota and
host
In mutualism, both organisms
benefit
In commensalism, one organism
benefits, and the other is
unaffected
In parasitism, the parasite
benefits at the damaging expense
of the host
Human host and microorganisms
Any parasite that causes disease is called
a pathogen
4. Normal microbiota of the human body
Normal microbiota
Colonize skin and mucous
membranes of the human body
without normally causing
disease
Resident microbiota
permanently colonize the
host
Transient microbiota may be
present for days, weeks, or
months
5. Members of the normal microbiota can become
opportunistic pathogens or opportunists
Members of the normal
microbiota gain access
to other body sites
Escherichia coli
(colon/mutualistic) can
cause urinary tract
infections (urethra)
Immune-
compromised/suppression
(disease, malnutrition, stress,
chemotherapy, immunosuppressive
drugs)
AIDS patients and fungal
infections (HIV infections
suppresses immune system
functions) – Candidiasis
Opportunistic members of the normal microbiota
6. The normal microbiota protect the host
Occupying niches that pathogens might occupy (nutrients)
Producing acids (vagina)
Producing bacteriocins that inhibit other bacteria
This is known as microbial antagonism or competitive
exclusion
Changes in the normal microbiota can lead to opportunistic
infections caused by members of the normal microbiota
Long-term antimicrobial treatment can cause changes in
the normal vaginal microbiota, leading to Candida albicans
infections
7. Reservoirs of infectious diseases
Reservoirs are sites where pathogens are maintained as
a source of infection
Human reservoirs
Animal reservoirs and zoonoses
Nonliving reservoirs
What is the source of pathogens?
8. Human reservoirs
People with active disease (signs/symptoms of a disease)
can transmit it
Carriers
People with asymptomatic disease or latent disease
are a source of infection
Gonorrhea
Typhoid fever
Syphilis
Tuberculosis
AIDS
Reservoirs of infectious diseases
9. Animal reservoirs (zoonoses)
Both wild and domestic animals are reservoirs of pathogens
Infectious diseases that spread from wild or domestic animals
to humans are called zoonoses
Transmission of zoonosis to humans
Direct contact with the infected animals or their waste
Consuming infected animal products
Contaminated food, water or air
Bloodsucking arthropods (vectors)
Usually, humans are a dead-end host for zoonotic pathogens
Zoonoses that involve bloodsucking arthropods can be
transmitted back to animal hosts
10.
11. Nonliving reservoirs
Nonliving reservoirs include
water, soil, foods (improperly
prepared or stored foods)
Water contaminated by
human/animal feces
Vibrio cholerae
Salmonella
Soil
Clostridium botulinum
C. tetani
12. How do pathogens enter the human body?
Pathogens enter the body at several sites called portals of entry
Three major types
Skin
A barrier to pathogens if it remains intact
Mucous membranes
Linings of the respiratory, gastrointestinal, urinary, and
reproductive tracts, conjunctiva
Placenta
Typically an effective barrier to most pathogens
13. Parenteral route: not a true portal of entry but allows
pathogens to circumvent the portals of entry
Pathogens gain direct access to tissues beneath the skin
and mucous membranes
Punctures by a nail or hypodermic needle
Sometimes included in this route:
breaks in the skin: stab wounds, bites,
surgery
Parenteral route
15. The pathogen must
Be transmitted to a susceptible
host
For an infectious disease to occur
There must be a reservoir of
infection as a source of pathogen
and a route of exit
Enter the host and attach to cells of the host (ligands)
Multiply sufficiently to adversely affect the body (causing
damages to the host)
To do so the pathogen must overcome the defense of the
host
16. Predisposing Factors
Predisposing factors make the body more susceptible to
disease
Inadequate nutrition
Fatigue
Age
Gender
Lifestyle
Chemotherapy
Emotional disturbances
17. Pathogens affect the host “producing virulence factors”
Virulence factors allow pathogens to enter a host, adhere to host
cells, or escape detection/killing by the immune system
Examples of virulence factors
Virulence factors Type
Ligands Adhesins, attachment proteins
Extracellular enzymes
Hyaluronidase, collagenase,
coagulase, kinases
Toxins
Endotoxins (Lipid A)
Exotoxins (cytotoxins, neurotoxins,
enterotoxins)
Anti-phagocytic factors Capsule, leukocidins
18. Some bacterial pathogens attach to the surface they colonize
and to each other to form a biofilm
Biofilms
Dental plaque
19. Extent of infections
Local infection: Pathogens are limited to a relatively small
area of the body (boils)
Systemic infection: An infection throughout the body
spread by the blood or lymph (measles)
Focal infection: Infection that “functions” as a source of
pathogen(s) to infections at other sites of the body (teeth)
20. Type of infections
Primary infection: Acute infection that causes the initial
illness
Secondary infection: Often opportunistic infections after
a primary (predisposing) infection – HIV and fungal
infections
Asymptomatic or subclinical infections: No noticeable
signs or symptoms – hepatitis
21. Extent of Host Involvement
Septicemia Systemic infection (blood)
Bacteremia Bacteria in the blood
Toxemia Toxins in the blood
Viremia Viruses in the blood
22. Manifestation of a disease
A disease causes changes in structures/functions of the
body
Symptom: A change in body function that is felt by a
patient such as pain or malaise (discomfort)
Sign: A change in the body that can be measured or
observed by a physician such as fever, swelling,
redness, diarrhea
Diagnosis
Involves an evaluation of signs, symptoms, and results
of laboratory tests
23. The Etiology of an infectious disease
Koch’s postulates help determine the etiology of an infectious
disease, which allows treatment and prevention
Exceptions to Koch’s postulates to
establish a single cause for certain infections
such as pneumonia
Ethical considerations
24. The Stages of an infectious disease
Many infectious diseases have five stages
25. Severity or Duration of a Disease
Acute disease
Disease develops rapidly/short time (common cold)
Chronic disease
Disease develops slowly, it is likely to continue or recur for long periods
(tuberculosis, hepatitis)
Subacute disease
Intermediate between acute and chronic
(bacterial endocarditis-streptococci)
Latent disease
A pathogen remains inactive for a long period before becoming active to
produce signs/symptoms of the disease (herpes/shingles - varicella virus)
26. Communicable disease: Any disease that is spread from
one host to another (genital herpes)
Contagious disease: A communicable disease that is easily
spread from one host to another (chickenpox)
Noncommunicable disease: A disease that
is not spread from one host to another
Clostridium tetani - abrasions or wounds
Types of Infectious Diseases
27. Pathogens must leave the infected patients in order to infect
other hosts
Portals of exit
Pathogens often leave the infected host via bodily secretions and
excretions
28. Transmission of an infectious disease
Transmission of a pathogen from a reservoir or a portal of exit
to another host’s portal of entry can occur through contact,
vehicles, vectors
Contact
Direct
Involves person-to-person contact, sexual intercourse
(gonorrhea), and transfer across the placenta
Animal to person contact, zoonosis (rabies/ring worm)
Indirect
Spread by fomites (non-living objects) - medical
equipment, needles (HIV virus)
29. Transmission of an infectious disease - Contact
Droplets (Distance < 1m)
Transmission via droplets
(droplets of mucus)
Cold and flu viruses
31. Vehicle transmission is the transmission of a pathogen by a medium
Air, water or foods
Blood or other bodily fluids handled outside the body
Airborne transmission
Transmission of an infectious disease
Transmission of pathogens via
aerosols (Distance >1m)
Aerosols from sneezing and
coughing , air conditioning
systems, flaming inoculating
loops
Mycobacterium tuberculosis, Legionella pneumophila
Fungal spores
32. Foodborne transmission
Waterborne transmission
Transmission of an infectious disease
Fecal-oral route of infection is a
major source of disease in the
world
Spread of gastrointestinal
diseases (giardiasis, cholera)
Contamination of foods with
feces and pathogens is another
example of fecal-oral
transmission
Food inadequately processed,
undercooked, poorly refrigerated
33. Vectors
Arthropods carry pathogens from one host to another
Fleas, ticks, mosquitoes, mites, bloodsucking flies
Transmit disease by 2 general methods:
Mechanical transmission: (passive process) - Arthropod carries
pathogen on feet or other body parts
Biological transmission: (active process) - Pathogen reproduces
within the vector and it is generally transmitted through a bite
Foodborne infections
Salmonella, Shigella, E. coli
35. Incidence
Fraction of a population that contracts a disease during a specific
time (new cases)
Prevalence
Fraction of a population having a specific disease at a given time
(total number of cases)
Occurrence and frequency of a disease -
Epidemiology
36. Sporadic disease: Disease that occurs occasionally in a
population
Endemic disease: Disease constantly present in a population
(common cold)
Epidemic disease: Disease acquired by many people in a given
area in a short time (greater frequency than usual)
Pandemic disease: Worldwide epidemic (influenza/AIDS!?)
Occurrence and frequency of a disease -
Epidemiology
38. Nosocomial Infections
Nosocomial Infections are acquired in hospitals, nursing homes
or other health care facilities
Nosocomial diseases
cause thousands deaths
annually in the US
Interaction between
three factors poses a
significant risk of
nosocomial infections
41. Microorganisms involved in nosocomial Infections
Clostridium difficile
Long-term antimicrobial
therapy in hospitalized patients
Excessive growth of C. difficile
(a transient microbe of the
colon)
Nosocomial diarrhea
Inflammation, bloody stools,
and formation of lesions in the
colon - pseudomembranous
colitis Lesions: connective tissue,
dying leukocytes, dead colon cells
42. Nosocomial infections are classified as
Exogenous
Pathogens acquired from the health care environment
Endogenous
Pathogens from the normal microbiota within the patient
Iatrogenic
Associated with modern medical procedures such as
catheters, invasive diagnostic procedures, and surgery
Nosocomial Infections
43. The Center for Disease Control and Prevention (CDC)
The CDC publishes Morbidity and Mortality Weekly Report (MMWR) , which
indicates the number of cases of the nationally notifiable diseases
Their occurrence must be reported to the CDC