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