Infection and Disease
The Host-Microbe Relationship
 Human body maintains a symbiosis with microbes
 Infection refers to:
 relationship between the host and microbe
 competition for supremacy between them
 If the host loses the competition, disease occurs
Contact, Colonization, Infection, Disease
 Infection
 condition where pathogenic microbes penetrate
host defenses
 enter tissues and multiply
 Disease
 any deviation from health, disruption of a tissue or
organ
 infectious disease
 Caused by microbes or their products
**Not all contacts lead to infections…….
**Not all infections lead to disease!!!!!
Resident Flora
 Microbiota, transient,
resident, indigenous
 Microbes that engage in
mutual or commensal
associations
 most areas of the body in
contact with the outside
environment
 internal organs & tissues &
fluids are microbe-free
 bacterial flora benefit host by
preventing overgrowth of
harmful microbes
Initial Colonization of the Newborn
 Uterus and contents normally
sterile
 remain so until just before birth
 Breaking of fetal membrane
exposes the infant
 all subsequent handling and
feeding continue to introduce
normal flora
Pathogens….
 True pathogens
 capable of causing disease in
healthy persons
 normal immune defenses
 influenza virus, plague bacillus,
malarial protozoan
 Opportunistic pathogens
 cause disease when hosts
defenses are compromised
 Pseudomonas sp & Candida
albicans
Pathogenicity
 Ability of a microorganism to
cause disease
 Severity of disease depends on
the virulence of the pathogen
 Degree of pathogenicity
 characteristic or structure that
contributes to the ability of a
microbe to cause disease
 virulence factor





Adhesion factors
Biofilms
Extracellular enzymes
Toxins
Antiphagocytic factors
Pathogens…..
 In order to become
established:
 Portal of entry
 Attaching
 Surviving host defenses
1. Portals of entry
 characteristic route a
microbe follows to enter the
tissues of the body
 skin
 nicks, abrasions, punctures,
incisions
 gastrointestinal tract
 food, drink, and other ingested
materials
 Mucous membranes
 urogenital tract
 Sexual
 transplacental
Infectious Dose (ID)
 minimum number of
microbes required for
infection to proceed
 microbes with small IDs have
greater virulence
 1 rickettsial cell in Q fever
 10 bacteria in TB, giardiasis
 109 bacteria in cholera
2. Mechanisms of Adhesion
 fimbrae
 flagella
 adhesive slimes or capsules
 cilia
 scolex
 hooks
 barbs
3. Surviving Host Defenses
 Initial response of host
defenses comes from
phagocytes
 Ability to survive
intracellular phagocytosis
 Antiphagocytic factors
Antiphagocytic factors
 Factors prevent phagocytosis by the host’s phagocytic
cells
 Bacterial capsule
 Composed of chemicals not recognized as foreign
 Slippery; difficult for phagocytes to engulf bacteria
 Antiphagocytic chemicals
 Prevent fusion of lysosome and phagocytic vesicles
 Leukocidins directly destroy phagocytic white blood cells
Enzymatic Destruction
 Extracellular enzymes (exoenzymes)
 Secreted by the pathogen
 Dissolve structural chemicals in the body
 Help pathogen maintain infection, invade, and avoid body defenses
Invasiveness
 Coagulase
 form a blood clot that protects them from phagocytosis
 Streptokinase
 dissolves fibrin clots and allows dissemination of the bacteria
 Hyaluronidase
 enhances pathogen penetration through tissues
 Leukocidins
 disintegrate neutrophils and macrophages
 Hemolysiins
 dissolve red blood cells
 Virulence can be enhanced in biofilms
 immune cells cannot reach bacterial cells
Toxins
 toxigenicity
 capacity to produce toxins at the site of multiplication
 endotoxins
 lipid A of LPS of gram-negative bacteria
 exotoxins
 proteins secreted by gram-positive and gram-negative bacteria
Toxins
 Toxinoses
 Disease caused by toxin
 Toxemias
 Spread by blood
 Intoxication
 Ingested
 Enterotoxins
 act on the gastrointestinal tract
 Antitoxins
 produced by the host body and
neutralize toxins
 Toxoids
 toxins whose toxicity has been
destroyed but still elicit an immune
response
Portals of Exit
 Respiratory, saliva
 Skin
 Fecal exit
 Urogenital tract
 Removal of blood
Establishment of Infection and Disease
 incubation period
 time between entry of the
microbe and symptom
appearance
 prodromal phase I
 time of mild signs or symptoms
 acme period (climax)
 signs and symptoms are most
intense
 period of decline
 As sign and symptoms subside
 period of convalescence
 body systems return to normal
Patterns of Infection
 localized infection
 remains confined to a specific tissue
 focal infection
 when infectious agent breaks loose from a local infection
 carried to other tissues
 systemic infection
 infection spreads to several sites and tissue fluids
 usually in the bloodstream
Patterns of Infection
 Mixed infection
 several microbes grow
simultaneously at the infection
site
 Primary infection
 initial infection
 Secondary infection
 another infection by a different
microbe
Warning Signals of Disease
 Sign
 objective evidence of disease as
noted by an observer
 Symptom
 subjective evidence of disease
as sensed by the patient
 Syndrome
 Disease is defined by complex
of signs and symptoms
Signs and Symptoms of
Inflammation
 Earliest symptoms of disease as a result of the
activation of the body defenses
 fever, pain, soreness, swelling
 Signs of inflammation:
 edema
 accumulation of fluid
 granulomas and abscesses
 walled-off collections of inflammatory cells
and microbes
 Lesion
 General term for site of infections or
disease
Signs of Infection in the Blood
 Changes in the number
of circulating white blood
cells
 leukocytosis
 increase in white blood
cells
 leukopenia
 decrease in white blood
cells
Signs of Infection in the Blood
 septicemia
 microorganisms multiplying in the blood and present
in large numbers
 bacteremia
 small numbers of bacteria present in blood
 not necessarily multiplying
 viremia
 small number of viruses present
 not necessarily multiplying
Infections That Go Unnoticed
 Asymptomatic
 subclinical infections
 although infected, the host doesn’t show any signs of
disease
 Inapparent infection
 person doesn’t seek medical attention
Persistence of Microbes and
Pathologic Conditions
 Latency
 microbe can periodically become active
 produce a recurrent disease
 person may or may not shed it during the latent stage
 Sequelae
 long-term or permanent damage to tissues or organs
 Acute diseases
 develop rapidly
 cause severe symptoms
 fade quickly
 Chronic diseases
 linger for long periods of time
 slower to develop and recede
Reservoirs: Where Pathogens
Persist
 Reservoir
 primary habitat of pathogen in the
natural world
 human or animal carrier, soil, water,
plants
 Source
 individual or object from which an
infection is actually acquired
Animals as Reservoirs and Sources
 vector
 live animal that transmits an
infectious agent from one host to
another
 Arthropods
 fleas, mosquitoes, flies, and ticks
 larger animals
 mammals, birds, lower vertebrates
Animals as Reservoirs and Sources
 zoonosis
 infection indigenous to animals
but naturally transmissible to
humans
 humans do not transmit the
disease to others
Acquisition and Transmission of
Infectious Agents
 Communicable disease
 an infected host can transmit the infectious agent to another
host
 Highly communicable disease is contagious
 Non-communicable infectious disease
 does not arise through transmission from host to host
 occurs primarily when a compromised person is invaded by
his or her own normal microflora
 contact with organism in natural, non-living reservoir
Patterns of
Transmission
 Direct contact
 physical contact, droplet nuclei,
aerosols
 Indirect contact
 passes from infected host to
intermediate conveyor and then to
another host
 vehicle
 food, water, biological products,
fomites
Nosocomial Infections
 Health care-associated
infections (HAIs)
 occur as a result of
receiving treatment for
another condition
 Nosocomial infections are
HAIs associated with
hospitals
 They occur as a result of
chains of transmission
Nosocomial Infections
 From surgical procedures,
equipment, personnel, and
exposure to drug-resistant
microorganisms
 More than 1/3rd of nosocomial
infections could be prevented
 2 to 4 million cases/year in U.S.
 approximately 90,000 deaths
 Most common organisms involved
 Gram-negative intestinal flora
 E. coli, Pseudomonas,
Staphylococcus
Epidemiology
 study of the frequency and distribution of
disease & health-related factors in human
populations
 Surveillance
 collecting, analyzing, & reporting data on rates of
occurrence, mortality, morbidity and transmission of
infections
 Reportable, notifiable diseases must be
reported to authorities
Epidemiology
 Centers for Disease Control and Prevention (CDC) in
Atlanta, GA
 principal government agency responsible for keeping track
of infectious diseases nationwide
 http://www.cdc.gov
Epidemiology
 Prevalence
 total number of existing cases with respect to the entire
population usually represented by a percentage of the
population
 Prevalence=
Total # of cases in pop
Total # of persons in pop
X 100
Epidemiology
 Incidence
 measures the number of new cases over a certain time
period, as compared with the general healthy population
 Incidence =
# of new cases
= ratio
# of healthy persons
The incidence and estimated
prevalence of AIDS
Epidemiology
 Mortality rate
 total number of deaths in a population due to a certain
disease
 Morbidity rate
 number of people afflicted with a certain disease
Epidemiology
 Endemic
 disease that exhibits a relatively
steady frequency over a long period of
time in a particular geographic locale
 Sporadic
 when occasional cases are reported at
irregular intervals
 Epidemic
 when prevalence of a disease is
increasing beyond what is expected
 Pandemic
 epidemic across continents