Chapter 14:Chapter 14:

Principles of Disease and Principles of Disease and EpidemiologyEpidemiology

IntroductionIntroduction PathologyPathology is the scientific study of disease. is the scientific study of disease.

EtiologyEtiology: Cause of disease.: Cause of disease.

PathogenesisPathogenesis: Studies how disease develops.: Studies how disease develops.

InfectionInfection: Invasion or colonization of the body : Invasion or colonization of the body

by pathogenic organisms.by pathogenic organisms.

DiseaseDisease: Occurs when an infection results in a : Occurs when an infection results in a

change from a state of health.change from a state of health.

Normal MicrobiotaNormal Microbiota Animals are generally free from microbes in utero.Animals are generally free from microbes in utero. After birth, microbial populations rapidly establish After birth, microbial populations rapidly establish

themselves in the newborn’s body.themselves in the newborn’s body. Vaginal lactobacilli colonize newborn’s intestine.Vaginal lactobacilli colonize newborn’s intestine. Feeding and breathing introduce many more microbes.Feeding and breathing introduce many more microbes.

E. coliE. coli and other bacteria colonize large intestine. and other bacteria colonize large intestine. Candida albicansCandida albicans colonizes mucous membranes. colonizes mucous membranes.

Normal Microbiota or FloraNormal Microbiota or Flora: Microorganisms that : Microorganisms that remain throughout an individual’s life.remain throughout an individual’s life.

Transient MicrobiotaTransient Microbiota: Microorganisms that are present : Microorganisms that are present for a certain time period and then disappear. for a certain time period and then disappear.

Cells in human body: Cells in human body: 1 x 101 x 101313

Microbes associated with human body: Microbes associated with human body: 1 x 101 x 101414

There are ~10 microorganisms/human body cell.There are ~10 microorganisms/human body cell.

Relationships Between the Normal Relationships Between the Normal Microbiota and the HostMicrobiota and the Host

Microbial AntagonismMicrobial Antagonism: Normal flora inhibits : Normal flora inhibits overgrowth of harmful microbes. overgrowth of harmful microbes.

Mechanisms include competition for nutrients Mechanisms include competition for nutrients and affecting environmental factors such as pH, and affecting environmental factors such as pH, toxic substances, and oxygen availability.toxic substances, and oxygen availability. Vaginal flora maintains pH of 3.5-4.5 which inhibits Vaginal flora maintains pH of 3.5-4.5 which inhibits

overgrowth of overgrowth of Candida albicansCandida albicans.. In mouth streptococci produce compounds that inhibit In mouth streptococci produce compounds that inhibit

growth of many other cocci.growth of many other cocci. In intestine In intestine E. coliE. coli produce produce bacteriocinsbacteriocins, which inhibit , which inhibit

growth of closely related bacteria.growth of closely related bacteria. C. dificileC. dificile is inhibited by normal intestinal flora. is inhibited by normal intestinal flora.

Relationships Between the Normal Relationships Between the Normal Microbiota and the HostMicrobiota and the Host

SYMBIOSISSYMBIOSIS: “Living together”. Relationship : “Living together”. Relationship between the host and its normal flora.between the host and its normal flora.

1. Commensalism1. Commensalism: One organism benefits, the : One organism benefits, the other is not affected (+/0). other is not affected (+/0). Many microbes live off secretions and dead cells and Many microbes live off secretions and dead cells and

do not benefit or harm host.do not benefit or harm host.

2. Mutualism2. Mutualism: Both organisms benefit from living : Both organisms benefit from living together (+/+).together (+/+). E. coli synthesizes vitamin K and some B vitamins.E. coli synthesizes vitamin K and some B vitamins.

3. Parasitism3. Parasitism: One organism benefits, the other is : One organism benefits, the other is harmed (+/-). harmed (+/-). Most disease causing bacteria.Most disease causing bacteria.

Opportunistic PathogensOpportunistic PathogensThe nature of symbiotic relationships can change.The nature of symbiotic relationships can change.

Opportunistic PathogensOpportunistic Pathogens: Organisms that : Organisms that normally do not cause disease in their natural normally do not cause disease in their natural habitat in a healthy person. They may cause habitat in a healthy person. They may cause disease if the host is weakened or if they enter a disease if the host is weakened or if they enter a different part of the body.different part of the body. Pneumocystis cariniiPneumocystis carinii pneumonia in AIDS patients. pneumonia in AIDS patients. Tooth decay and gum disease caused by mouth flora.Tooth decay and gum disease caused by mouth flora. Neisseria meningitidisNeisseria meningitidis is usually harmless in is usually harmless in

respiratory tract, but can cause meningitis.respiratory tract, but can cause meningitis. E. coliE. coli can cause urinary tract infections, meningitis, can cause urinary tract infections, meningitis,

pneumonia, and abscesses.pneumonia, and abscesses.

Cooperation Among MicrobesCooperation Among MicrobesSynergismSynergism: The effect of two microbes acting : The effect of two microbes acting

together, is greater than the effect of either acting together, is greater than the effect of either acting alone.alone.

AIDS and mycoplasma infection: Cells infected with AIDS and mycoplasma infection: Cells infected with mycoplasma and HIV die more readily than those infected mycoplasma and HIV die more readily than those infected with mycoplasma alone.with mycoplasma alone.

AIDS and Oncogenic Viruses: AIDS and Oncogenic Viruses: Women with HIV infections develop very aggressive cervical Women with HIV infections develop very aggressive cervical

cancers which are caused by papillomavirus.cancers which are caused by papillomavirus. Individuals with HIV and Human Herpes Virus 8 infections, are Individuals with HIV and Human Herpes Virus 8 infections, are

more likely to develop Kaposi’s sarcoma.more likely to develop Kaposi’s sarcoma.

Oral streptococci and pathogens that cause gingivitis and Oral streptococci and pathogens that cause gingivitis and periodontal disease. Pathogens bind to streptococci periodontal disease. Pathogens bind to streptococci instead of host tissue.instead of host tissue.

Etiology of Infectious DiseasesEtiology of Infectious DiseasesDiseases can be caused by many factors: infection, Diseases can be caused by many factors: infection, genetics, degeneration, and others.genetics, degeneration, and others.

Koch’s PostulatesKoch’s Postulates

Developed by Robert Koch in 1877 to establish Developed by Robert Koch in 1877 to establish cause of infectious diseases: anthrax and TB. cause of infectious diseases: anthrax and TB.

1. Same pathogen must be present in every case of the 1. Same pathogen must be present in every case of the disease.disease.

2. Pathogen must be isolated from diseased host and grown 2. Pathogen must be isolated from diseased host and grown in pure culture.in pure culture.

3. Pathogen from pure culture must cause disease when 3. Pathogen from pure culture must cause disease when inoculated in healthy, susceptible laboratory animal.inoculated in healthy, susceptible laboratory animal.

4. Pathogen must be isolated from inoculated animal and 4. Pathogen must be isolated from inoculated animal and shown to be the original organism.shown to be the original organism.

Koch’s Postulates for Infectious Diseases

Exceptions to Koch’s PostulatesExceptions to Koch’s Postulates

Koch’s principles do not apply to all diseases. Koch’s principles do not apply to all diseases.

1. Some microbes cannot be cultured in artificial media.1. Some microbes cannot be cultured in artificial media. Treponema pallidumTreponema pallidum (syphillis) (syphillis) Mycobacterium lepraeMycobacterium leprae (leprosy) (leprosy) Ricketsias, chlamydias, and viruses only multiply within cells.Ricketsias, chlamydias, and viruses only multiply within cells.

2. One disease may involve several different pathogens.2. One disease may involve several different pathogens. DiarrheaDiarrhea PneumoniaPneumonia MeningitisMeningitis PeritonitisPeritonitis NephritisNephritis

3. Some pathogens may cause several different diseases.3. Some pathogens may cause several different diseases. Streptococcus pyogenesStreptococcus pyogenes: Scarlet fever, sore throat, skin infections, : Scarlet fever, sore throat, skin infections,

bone infections, etc.bone infections, etc. Mycobacterium tuberculosisMycobacterium tuberculosis: Causes disease of lungs, skin, bones, : Causes disease of lungs, skin, bones,

and internal organs.and internal organs.

Effects of Infectious Diseases Effects of Infectious Diseases Diseases alter normal body function and/or Diseases alter normal body function and/or structure.structure.

SymptomsSymptoms: Subjective feelings not obvious to an : Subjective feelings not obvious to an observer. observer.

ExampleExample: Pain and malaise.: Pain and malaise. SignsSigns: Objective changes that can be measured.: Objective changes that can be measured.

ExampleExample: Fever, redness, swelling, paralysis.: Fever, redness, swelling, paralysis. SyndromeSyndrome: A group of signs and symptoms that : A group of signs and symptoms that

are associated with a disease.are associated with a disease.

ExampleExample: AIDS (Acquired Immune Deficiency : AIDS (Acquired Immune Deficiency Syndrome) is a syndrome that varies from one Syndrome) is a syndrome that varies from one individual to another.individual to another.

Classifying Infectious DiseasesClassifying Infectious Diseases Communicable DiseasesCommunicable Diseases: Spread from one host to : Spread from one host to

another, directly or indirectly.another, directly or indirectly.

Example: Tuberculosis, herpes, flu, AIDS, Example: Tuberculosis, herpes, flu, AIDS, chickenpox, mumps, polio, and hepatitis.chickenpox, mumps, polio, and hepatitis.

Contagious DiseasesContagious Diseases: Spread easily from one : Spread easily from one person to another.person to another.

Example: Chickenpox and measles.Example: Chickenpox and measles. Noncommunicable DiseasesNoncommunicable Diseases: Not spread from one : Not spread from one

host to another. Caused by microbes that live host to another. Caused by microbes that live outside the body or by opportunistic pathogens outside the body or by opportunistic pathogens that live inside the body.that live inside the body.

Example: Tetanus, botulism, and yeast infections. Example: Tetanus, botulism, and yeast infections.

Disease OccurrenceDisease Occurrence Disease IncidenceDisease Incidence: Percentage of population that : Percentage of population that

contractscontracts a disease in a given time period. a disease in a given time period. Disease PrevalenceDisease Prevalence: Percentage of population : Percentage of population

that that hashas the disease during given time period. the disease during given time period. Sporadic DiseaseSporadic Disease: Occurs only occasionally.: Occurs only occasionally.

Example: Polio in U.S.Example: Polio in U.S. Endemic DiseaseEndemic Disease: Constantly present in the population.: Constantly present in the population.

Example: Common cold or ear infections.Example: Common cold or ear infections. Epidemic DiseaseEpidemic Disease: Many people acquire disease in short : Many people acquire disease in short

time period.time period.

Example: Influenza, gonorrhea, chlamydia, and AIDS.Example: Influenza, gonorrhea, chlamydia, and AIDS. Pandemic DiseasePandemic Disease: Worldwide epidemics.: Worldwide epidemics.

Example: Influenza and AIDS. Example: Influenza and AIDS.

Reported AIDS Cases in the United States

Disease DurationDisease Duration Acute DiseaseAcute Disease: Develops rapidly, but lasts a short : Develops rapidly, but lasts a short

time. time. Examples: Flu and common cold.Examples: Flu and common cold.

Chronic DiseaseChronic Disease: Develops more slowly, and reactions : Develops more slowly, and reactions are less severe. Tend to recur for long periods or to be are less severe. Tend to recur for long periods or to be continual. continual. Examples: Tuberculosis, hepatitis B, and infectious Examples: Tuberculosis, hepatitis B, and infectious

mononucleosis.mononucleosis.

Subacute DiseaseSubacute Disease: Intermediate between acute and : Intermediate between acute and chronic.chronic. Examples: Subacute bacterial endocarditis (streptococci). Examples: Subacute bacterial endocarditis (streptococci).

Latent DiseaseLatent Disease: Causative agent remains inactive for : Causative agent remains inactive for a time, but then becomes active and produces disease a time, but then becomes active and produces disease symptoms.symptoms. Examples: Shingles, genital and oral herpes, AIDS.Examples: Shingles, genital and oral herpes, AIDS.

Host InvolvementHost Involvement Local InfectionLocal Infection: Microbes are limited to small : Microbes are limited to small

area of body.area of body.Examples: Boils, abscesses, and acne.Examples: Boils, abscesses, and acne.

Systemic (Generalized) InfectionSystemic (Generalized) Infection: Microbes are : Microbes are spread throughout body by blood or lymph.spread throughout body by blood or lymph.Examples: Measles, AIDS.Examples: Measles, AIDS. BacteremiaBacteremia: Presence of bacteria in the blood.: Presence of bacteria in the blood. SepticemiaSepticemia: Bacteria multiply in blood.: Bacteria multiply in blood. ToxemiaToxemia: Presence of toxins in blood.: Presence of toxins in blood. ViremiaViremia: Presence of viruses in blood.: Presence of viruses in blood.

Focal InfectionFocal Infection: Starts as a local infection and : Starts as a local infection and spread to other parts of body.spread to other parts of body.Examples: From teeth, tonsils, and appendix.Examples: From teeth, tonsils, and appendix.

Host Resistance Determines Extent of InfectionHost Resistance Determines Extent of Infection Primary InfectionPrimary Infection: Acute infection that causes : Acute infection that causes

initial illness.initial illness.

Example: Common coldExample: Common cold Secondary InfectionSecondary Infection: Caused by opportunistic : Caused by opportunistic

pathogen after primary infection has weakened host pathogen after primary infection has weakened host immune system.immune system.

Example: Pneumonia or bronchitis may develop Example: Pneumonia or bronchitis may develop after the common cold.after the common cold.

Subclinical InfectionSubclinical Infection: Does not cause any noticeable : Does not cause any noticeable illness in host.illness in host.

Example: Over 90% of polio infections are Example: Over 90% of polio infections are asymptomatic.asymptomatic.

Spread of InfectionSpread of InfectionReservoirs of InfectionReservoirs of Infection Human ReservoirsHuman Reservoirs: Infected individuals who may : Infected individuals who may

or may not present disease. or may not present disease. CarriersCarriers are infected are infected individuals without any signs or symptoms of individuals without any signs or symptoms of disease (AIDS, polio, gonorrhea).disease (AIDS, polio, gonorrhea).

Animal ReservoirsAnimal Reservoirs: Zoonoses are diseases that : Zoonoses are diseases that occur primarily in wild and domestic animals. occur primarily in wild and domestic animals. About 150 different zoonoses are known (rabies, About 150 different zoonoses are known (rabies, anthrax, and Lyme disease).anthrax, and Lyme disease).

Nonliving ReservoirsNonliving Reservoirs: Two major sources are soil : Two major sources are soil and water.and water. Soil: Soil: Clostridium tetaniClostridium tetani and and botulinumbotulinum.. Water: Water: Vibrio choleraeVibrio cholerae and and Salmonella typhiSalmonella typhi..

Transmission of DiseaseTransmission of DiseaseI. Contact TransmissionI. Contact Transmission: Spread by direct contact, : Spread by direct contact,

indirect contact, or droplet transmission.indirect contact, or droplet transmission.

A. Direct Contact TransmissionA. Direct Contact Transmission: Person-to-person : Person-to-person transmission. No intermediate object is involved. transmission. No intermediate object is involved. Examples: Touching, kissing, sexual intercourse.Examples: Touching, kissing, sexual intercourse.

B. Indirect Contact TransmissionB. Indirect Contact Transmission: Agent is transferred : Agent is transferred via a nonliving object (via a nonliving object (fomitefomite).).

Examples: Towels, eating utensils, thermometers, Examples: Towels, eating utensils, thermometers, stethoscopes, bedding, clothes, money, and syringes. stethoscopes, bedding, clothes, money, and syringes.

C. Droplet TransmissionC. Droplet Transmission: Microbes are spread in mucus : Microbes are spread in mucus droplets that travel droplets that travel short distancesshort distances ( (less than 1 meterless than 1 meter). ). Examples: Sneezing, coughing, talking, and laughing.Examples: Sneezing, coughing, talking, and laughing.

Transmission of Disease (Continued)Transmission of Disease (Continued)II. Vehicle TransmissionII. Vehicle Transmission: Transmission of disease : Transmission of disease

via medium such as water, food, air, blood, body via medium such as water, food, air, blood, body fluids, and intravenous fluids.fluids, and intravenous fluids. Waterborne TransmissionWaterborne Transmission: Usually caused by water : Usually caused by water

contaminated with sewage.contaminated with sewage. Airborne TransmissionAirborne Transmission: Spread of agents by droplets : Spread of agents by droplets

in dust that in dust that travel more than 1 mtravel more than 1 m to host. to host.

III. VectorsIII. Vectors: Animals that carry disease from one : Animals that carry disease from one host to another. Arthropods (insects) are most host to another. Arthropods (insects) are most important animal vectors.important animal vectors. Mechanical TransmissionMechanical Transmission: Passive transport of : Passive transport of

pathogens on insect’s body.pathogens on insect’s body. Biological TransmissionBiological Transmission: Pathogen spends part of its : Pathogen spends part of its

life cycle in the vector.life cycle in the vector.

Portals of ExitPortals of ExitSite at which microbes leave body. Most common Site at which microbes leave body. Most common exit portals are respiratory and gastrointestinal exit portals are respiratory and gastrointestinal tracts.tracts.

Respiratory TractRespiratory Tract: Exit in discharges (mucus : Exit in discharges (mucus droplets) from nose and mouth. Transmission by droplets) from nose and mouth. Transmission by coughing, sneezing, spitting, etc.coughing, sneezing, spitting, etc. Examples: Tuberculosis, influenza, pneumonia, Examples: Tuberculosis, influenza, pneumonia,

common cold, measles, mumps, scarlet fever, and common cold, measles, mumps, scarlet fever, and meningococcal meningitis.meningococcal meningitis.

Gastrointestinal TractGastrointestinal Tract: Exit in feces or saliva.: Exit in feces or saliva. Feces: Amoebas, poliovirus, cholera, typhoid fever, Feces: Amoebas, poliovirus, cholera, typhoid fever,

salmonella, shigella, and many helminths.salmonella, shigella, and many helminths. Saliva: Rabies virus, herpes simplex 1.Saliva: Rabies virus, herpes simplex 1.

Portals of Exit (Continued)Portals of Exit (Continued) Urogenital TractUrogenital Tract: Exit in secretions or urine.: Exit in secretions or urine.

Penis and vagina: Sexually transmitted diseases. Penis and vagina: Sexually transmitted diseases. Chlamydia, herpes simplex 2, HIV, gonorrhea, and Chlamydia, herpes simplex 2, HIV, gonorrhea, and syphilis.syphilis.

Urine: Brucellosis and typhoid fever.Urine: Brucellosis and typhoid fever. Skin and Wound InfectionsSkin and Wound Infections: Spread through : Spread through

direct contact or through fomites.direct contact or through fomites. Example: Example: S. aureus, P. aeruginosaS. aureus, P. aeruginosa, scabies, , scabies,

ringworm.ringworm. BloodBlood: Transmission through insects, needles, : Transmission through insects, needles,

and syringes.and syringes. Insects: Malaria, yellow fever, and Lyme disease.Insects: Malaria, yellow fever, and Lyme disease. Needles: AIDS and hepatitis B.Needles: AIDS and hepatitis B.

Nosocomial (Hospital Acquired) InfectionsNosocomial (Hospital Acquired) Infections ““Nosocomial”Nosocomial” Greek word for hospital. Greek word for hospital. Infections acquired at a health care facility.Infections acquired at a health care facility. According to According to Center for Disease ControlCenter for Disease Control (CDC), (CDC), 5-5-

15%15% of all hospital patients acquire N.I.s. of all hospital patients acquire N.I.s. At least At least 100,000 deaths100,000 deaths in US from N.I.s every year. in US from N.I.s every year. Predisposing Factors:Predisposing Factors:

Wide variety of microbes in hospital environmentWide variety of microbes in hospital environment Weakened or immunocompromised patientsWeakened or immunocompromised patients Chain of transmission: Mainly through direct or Chain of transmission: Mainly through direct or

indirect contact.indirect contact.• From health care workers to patientFrom health care workers to patient• From patient to patientFrom patient to patient• Fomites: Catheters, needles, dressings, beds, wheelchairsFomites: Catheters, needles, dressings, beds, wheelchairs• Airborne transmissionAirborne transmission

Important Nosocomial PathogensImportant Nosocomial Pathogens Normal microbiota:Normal microbiota: Many are opportunistic Many are opportunistic

pathogenspathogens Antibiotic resistanceAntibiotic resistance: Very high due to the use of : Very high due to the use of

antimicrobials in health care facilities.antimicrobials in health care facilities. Principle microorganismsPrinciple microorganisms: Used to be gram-: Used to be gram-

positive microbes. Today positive microbes. Today mostmost are are gram-negative gram-negative bacteria.bacteria. EnterobacteriaEnterobacteria: Over 40% of all infections. : Over 40% of all infections. E. coli, E. coli,

Klebsiella spp., Proteus spp., Enterobacter sppKlebsiella spp., Proteus spp., Enterobacter spp., and ., and Serratia marcescensSerratia marcescens..

Staphylococcus aureusStaphylococcus aureus (11%)(11%) FungiFungi: (10%) : (10%) C. albicansC. albicans and others and others EnterococcusEnterococcus (10%)(10%) Pseudomonas aeruginosaPseudomonas aeruginosa (9%) (9%)

Principal Kinds of Nosocomial InfectionsPrincipal Kinds of Nosocomial Infections

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Control of Nosocomial InfectionsControl of Nosocomial Infections Aseptic techniques to avoid contaminationAseptic techniques to avoid contamination Careful handling and disposal of contaminated Careful handling and disposal of contaminated

materialmaterial Frequent and adequate hand washingFrequent and adequate hand washing Proper infection control training of staffProper infection control training of staff Isolation wards and roomsIsolation wards and rooms Avoid unnecessary antibiotic prescriptionsAvoid unnecessary antibiotic prescriptions Avoid unnecessary invasive proceduresAvoid unnecessary invasive procedures Regular disinfection of respirators and Regular disinfection of respirators and

humidifiers and maintenance of autoclaveshumidifiers and maintenance of autoclaves Use disposable and/or sterile suppliesUse disposable and/or sterile supplies Infection control committeeInfection control committee

Predisposing Factors for DiseasePredisposing Factors for DiseaseFactors that make an individual more susceptible Factors that make an individual more susceptible to a given disease.to a given disease.

GenderGender Genetic factorsGenetic factors Climate and weatherClimate and weather NutritionNutrition Fatigue/StressFatigue/Stress OccupationOccupation EnvironmentEnvironment LifestyleLifestyle AgeAge Other illnessOther illness

Development of DiseaseDevelopment of Disease1. Incubation Period1. Incubation Period: Time between initial : Time between initial

infection and appearance of signs and symptomsinfection and appearance of signs and symptoms2. Prodromal Period2. Prodromal Period: Early, mild symptoms of : Early, mild symptoms of

disease.disease.3. Illness Period3. Illness Period: Disease is most acute. Overt : Disease is most acute. Overt

signs and symptoms. Patient immune system signs and symptoms. Patient immune system actively fights off infection. If not successful may actively fights off infection. If not successful may die at this stage.die at this stage.

4. Decline Period4. Decline Period: Signs and symptoms subside. : Signs and symptoms subside. Patient is vulnerable to secondary infections.Patient is vulnerable to secondary infections.

5. Convalescence Period5. Convalescence Period: Recovery. Body returns : Recovery. Body returns to predisease state.to predisease state.

The Stages of a Disease

EpidemiologyEpidemiology Science that studies when and where diseases Science that studies when and where diseases

occur and how they are transmitted in a occur and how they are transmitted in a population.population.

Modern epidemiology began in 1848 during a Modern epidemiology began in 1848 during a cholera epidemic in London.cholera epidemic in London.

Epidemiologists collect information to determine:Epidemiologists collect information to determine: EtiologyEtiology: Cause of disease (pathogen).: Cause of disease (pathogen). Predisposing factorsPredisposing factors: Age, sex, lifestyle, etc.: Age, sex, lifestyle, etc. IncidenceIncidence: Number of individuals acquiring disease in : Number of individuals acquiring disease in

a given time period.a given time period. PrevalencePrevalence: Number of individuals with disease in a : Number of individuals with disease in a

given time period.given time period. TransmissionTransmission Public Health Policy and PreventionPublic Health Policy and Prevention

Types of Epidemiological InvestigationTypes of Epidemiological Investigation Descriptive EpidemiologyDescriptive Epidemiology: Retrospective studies : Retrospective studies

that collect data on diseases occurence.that collect data on diseases occurence. Examples: Initial studies on toxic shock syndrome Examples: Initial studies on toxic shock syndrome

outbreaks.outbreaks. Analytical EpidemiologyAnalytical Epidemiology: Analyzes a disease to : Analyzes a disease to

determine its probable cause.determine its probable cause. Case control methodCase control method: Look for factors that precede : Look for factors that precede

disease. Compare sick individuals to healthy disease. Compare sick individuals to healthy individuals.individuals.

Cohort methodCohort method: Study two populations, one that has : Study two populations, one that has been exposed to a certain risk factor (e.g.: blood been exposed to a certain risk factor (e.g.: blood transfusion or sexual contact) . Control group is not transfusion or sexual contact) . Control group is not exposed to risk factor.exposed to risk factor.

Experimental EpidemiologyExperimental Epidemiology: Test drugs or : Test drugs or vaccines on susceptible populations.vaccines on susceptible populations.

Case ReportingCase ReportingProcedure that requires health workers to report Procedure that requires health workers to report

specifiedspecified diseases to government agencies. diseases to government agencies.

Partial listPartial list of reportable diseases in the U.S.: of reportable diseases in the U.S.: AIDSAIDS BotulismBotulism ChlamydiaChlamydia DiphtheriaDiphtheria GonorrheaGonorrhea Hepatitis A, B, and CHepatitis A, B, and C Pediatric HIV infectionPediatric HIV infection MalariaMalaria TetanusTetanus TuberculosisTuberculosis

Center for Disease Prevention and Center for Disease Prevention and Control (CDC)Control (CDC)

Central source of epidemiological information in Central source of epidemiological information in U.S.U.S.

Branch of U.S. Public Health ServiceBranch of U.S. Public Health Service Located in Atlanta, GeorgiaLocated in Atlanta, Georgia Weekly publication “Morbidity and Mortality Weekly publication “Morbidity and Mortality

Report” (MMWR)Report” (MMWR) MorbidityMorbidity: Number of specific notifiable diseases.: Number of specific notifiable diseases. MortalityMortality: Number of deaths from diseases.: Number of deaths from diseases. Reports on disease outbreaksReports on disease outbreaks Special interest case historiesSpecial interest case histories Summaries on status of diseasesSummaries on status of diseases Recommendations for diagnosis, immunization, and Recommendations for diagnosis, immunization, and

treatment.treatment.