Disease Process

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• Relationship between 2 or more species • Mutualism- both benefit, can’t survive without each other
• Not true mutualism, not essential

• Commensalism-one benefits and other unaffected • Long lasting and stable relationship • Result of long term evolution

• One benefits at expense of other
• • • • Pathogen-causes disease Metabolically unstable relationship Host dies or eliminates pathogen Most successful parasites

• Host under continuous pressure from infection

• Battle between host & parasite

• Emergence of new & return of old infectious diseases

• 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

• Intensity of disease produced
• Animal passage- bug becomes more virulent with each transfer through same species

• Weakening of disease producing ability of pathogen • Transposal of virulence

• Contamination • Infection- multiples and invades tissue • Disease- disrupts state of health

Normal Flora
• 1 x10 13 body cells with 1 X1014 bacterial cell • Viruses, fungi, protozoa not found in high #s • Resident flora always present

Normal Flora
• Resident flora

• Transient flora

• Usually cause no disease • Conditions allow them to cause disease

• Intro of bug into unusual body sites
• Disturbance of normal flora

• Presence of bugs, multiplication but no invasion
• Nose & throat-staph, strep, Neisseria, yeastpotential pathogens • Skin-staph, yeast, diphtheroids • Colon-viruses, E. coli, enterobacter • Urinary and genital systems • Vagina-yeast and bacteria

Advantage of Normal Flora
• Ensures normal development of immune system • Prevent over growth of harmful organismmicrobial 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

• Cause of disease • Koch’s postulates-organism causes disease

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 • Host response
– Headache – Aches & pain

Invasive Period
• Period of illness-most acute • Overt signs and symptoms • Cough, sore throat

• S&S subside • Regain strength and recovery

• Study of mechanism and factors involved in the frequency and spread of disease • Incidence of diseases • Prevalence of diseases

Chain of Infection
Elderly & Young Pts w/ Chronic Illness Diabetic Use of Invasive Equip

Susceptible Host

Infecting Agents
Bacteria Parasites Viruses

People Food Equipment


Broken Skin Respiratory System GI & GU System Direct Contact Indirect Contact

Portal of Exit

Respiratory System GI & GU System

Airborne Droplet

Spread of Disease
• • • • Chain of infection Agent- pathogen Reservoir-source/site of organism Human reservoirs

• Animal reservoirs -zoonoses

• 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

• • • • • 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 • Adapt to 2 different environments • 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

• • • • Large droplets, short distances Mucous droplets -coughing , sneezing Pertussis, influenza, SARS Talking less transmission

• Direct- person to person, touching ,sex, colds
– Horizontal transmission – Fecal-oral transmission especially if public health & hygiene lacking

• 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

• 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

Nosocomial Infections
• Healthcare acquired
– Exogenous

• 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

• SSIs –prophylactic antibiotics
• Devices- central lines & ventilators

• Quarantine
• Immunization- influenza & pneumococcal • Vector control

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