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PRINCIPLES OF DISEASE
Symbiosis
• Relationship between 2 or more species
• Mutualism- both benefit,
Symbiosis
• Commensalism-one benefits and other unaffected
Parasitism
• One benefits at expense of other
• Pathogen-causes disease
• Metabolically unstable relationship
• Host dies or eliminates pathogen
• Most successful parasites
Parasitism
• Host under continuous pressure from infection
• Battle between host & parasite
• Emergence of new & return of old infectious diseases
Pathogenicity
• Capacity to produce disease– Adherence– Multiply on host– Ability to invade host or cause damage– Avoid being damaged by host’ defenses– Depends upon # of organisms
Virulence
• Intensity of disease produced
Terms
• Contamination
• Infection- multiples and invades tissue
• Disease- disrupts state of health
Normal Flora
• 1 x10 13 body cells with 1 X1014 bacterial cell
• Resident flora always present
Normal Flora
• Resident flora
• Transient flora
Opportunists
• Usually cause no disease
• Conditions allow them to cause disease
Opportunists
• Intro of bug into unusual body sites
• Disturbance of normal flora
Advantage of Normal Flora
• Ensures normal development of immune system
• Prevent over growth of harmful organism-microbial antagonism
Microbial Antagonism
• Normal flora in colon prevents overgrowth of C. difficile
• E. coli produces bacteriocins
Disadvantage of Normal Flora
• Potential for spread into sterile parts of body
• Intestine may perforate
• Skin broken
• Extraction of tooth
• Perianal skin flora enters urinary tract
Etiology
• Cause of disease• Koch’s postulates-organism causes disease
Frequency of Disease
• Endemic– Always present in population
• Epidemic– Occurs in unusually high number of people
• Pandemic-– Epidemics world wide
Types of Infectious Diseases
• Acute -develops rapidly but lasts for short time- influenza
• Chronic disease -develops more slowly and continues or recurs for long periods -TB, hepatitis B
• Latent- agent remains inactive for a time and later becomes active-shingles
Herd Immunity
• Proportion of people in community who are immune
• Important in cyclic diseases• If high, disease can only spread among
susceptible people
• Loss of herd can lead to reemergence of disease
Extent of Disease
• Local infection-limited to small area of body- boils
• Focal - starts as local infection (sinus or teeth) then enter blood or lymph and spreads
• Systemic -organisms or products spread through out body-measles
• Bacteremia- presence of bacteria in blood
Extent of Disease
• Septicemia-bacteria multiplies in blood
• Toxemia-toxins in blood
• Viremia-virus in blood
• Subclinical (inapparent )- no noticeable illness-hepatitis A
• Primary disease- initial acute infection
• Secondary infection -opportunistic infection
Stages of Disease
• Incubation period-time between initial infection and first appearance of S&S
Prodromal Period
• Short period–only in some diseases
Invasive Period
• Period of illness-most acute
• Overt signs and symptoms
• Cough, sore throat
Decline/Convalescence
• S&S subside
• Regain strength and recovery
Epidemiology
• Study of mechanism and factors involved in the frequency and spread of disease
• Incidence of diseases
• Prevalence of diseases
People FoodEquipment
Entry
Transmission
Reservoir
Portal ofExit
InfectingAgentsSusceptibl
e Host Bacteria Parasites Viruses
Respiratory SystemGI & GU System
Direct ContactIndirect Contact
AirborneDroplet
Broken Skin Respiratory System
GI & GU System
Elderly & Young
Pts w/ Chronic Illness
Diabetic
Use of Invasive Equip
Chain of Infection
Spread of Disease
• Chain of infection• Agent- pathogen• Reservoir-source/site of organism• Human reservoirs
• Animal reservoirs -zoonoses
Humans
• Sick people
• Carriers– Incubatory or asymptomatic carriers
• HIV but not AIDS• Hepatitis C
– Chronic carriers• Typhoid Mary excreted salmonella for years in feces• S. pyogenes in throat
Animals
• Domestic and wild
• Mammals carry rabies-exposure to saliva
• Consume contaminated animals or products
• Arthropod borne-West Nile
• Zoonoses– Lyme disease: wild deer and mice– Hantavirus pulmonary disease: rodents
Environmental Reservoirs
• Able to survive in nonliving reservoirs
• Soil: C. tetani – Humans produces toxin– Survives in soil by forming endospores
• Contaminated water
Portal of Exit
• Via body fluid or feces– Respiratory tract– GI – GU– Nonintact skin-lesions, wounds
Modes of Transmission
• Airborne– Tiny droplet nuclei vs large droplets– Dust particles– Suspended in air don’t fall– More likely to reach lower resp tract– Resistant to drying
• TB, measles and chicken pox
– Spread rapidly in crowded conditions
Droplet
• Large droplets, short distances
• Mucous droplets -coughing , sneezing
• Pertussis, influenza, SARS
• Talking less transmission
Contact
• Direct- person to person, touching ,sex, colds– Horizontal transmission – Fecal-oral transmission especially if public
health & hygiene lacking
Contact
• Vertical– Parent to offspring-birth canal, breast milk,
placenta
• Indirect-via fomites-tissues, diapers , door knobs- hands– Normal person sheds skin atrr rate of 5 x10 8
per day
• Hep B, C, D, lice, STDs
Vehicle
• Via medium-water, food, blood - Shigella in water or food, S. aureus
• Vector-arthropod– Mechanical-passive – Biological- active
•
Portals of Entry
• Respiratory
• GI - in food and water
• GU-sexually transmitted microbes
• Non intact skin- parenteral
Susceptible Host
• Imunocompromised
• Old age or young
• Not vaccinated
• Large inoculum
Healthcare Infections
• Healthcare acquired– Exogenous
– Endogenous
Consequences
• Serious illness or death
• Prolonged hospital stay
• Need for antimicrobial therapy
• Foci for spreading infection
Controlling Disease Transmission
• Standard precautions-everyone
• Isolation for communicable diseases or bugs
Prevention
• SSIs –prophylactic antibiotics
• Devices- central lines & ventilators
Prevention
• Quarantine
• Immunization- influenza & pneumococcal
• Vector control