1. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Chapter 13 Microbe-Human Interactions Infection, Disease, and Epidemiology 1

2. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 2 We Are Not Alone The human body exists in a state of dynamic equilibrium Many interactions between human body and microorganisms involve the development of biofilms Colonization of the body involves a constant “give and take”

3. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. We are not alone Human-microbe interactions lead to: microbial stabilization of body surfaces and protection from infections maturation of host defenses and development of the immune system invasion by microbes and their growth in sterile tissues disease caused by microbial damage to tissues and organs 3

4. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 4 Contact, Colonization, Infection, Disease Microbes that engage in mutual or commensal associations – normal resident microbiota (aka: indigenous microflora, normal flora, commensals) Infection – a condition in which pathogenic microbes penetrate host defenses, enter sterile tissues, and multiply. Pathogen – infectious agent Infectious disease – an infection that causes damage or disruption to tissues and organs

5. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Resident Flora Most areas of the body in contact with the outside environment harbor resident microbes e.g: ear, eyes, GIT Internal organs, tissues, and fluids are microbe-free E.g: LIVER, KIDNEY, LUNGS, BRAIN, HEART, BONES, MUSCLES, BLOOD, URINE(B4 URETHRA), CSF, SALIVA(B4 THE MOUTH), AMNIOTIC FLUID, BABY Transients – microbes that occupy the body for only short periods Residents – microbes that become established 5

6. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 6 Resident Flora Bacterial flora benefit host by preventing overgrowth of harmful microbes – microbial antagonism E.g: Lactobacilli prevents overgrowth of Candida albicans. E.coli prevents Salmonella and Shigella Endogenous infections – occur when normal flora is introduced to a site that was previously sterile E.g: E. coli from large intestine gets into the bladder AIDS patients easily infected by microflora Copyright © McGraw-Hill Education. Permission required for reproduction or display.

7. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 7 Initial Colonization of the Newborn Uterus and contents are normally sterile and remain so until just before birth Breaking of fetal membrane exposes the infant; all subsequent handling and feeding continue to introduce what will be normal flora. Exposure to microbes is unavoidable and necessary Exposure to pathogens is dangerous Copyright © McGraw-Hill Education. Permission required for reproduction or display. Streptococci, staphylococci, and lactobacilli, Bifidobacterium (breast fed)

8. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 8 Indigenous Flora of Specific Region

9. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 9 Flora of the Human Skin Skin is the largest and most accessible organ Norma flora lives in/or on dead cell layers, follicles and glands Two cutaneous populations –Transients: influenced by hygiene –Residents: stable, predictable, less influenced by hygiene Duct of sudoriferous (sweat) gland Subcutaneous tissue (a) Hair root Hair follicle Sebaceous (oil) gland Sweat pores Hair shaft Dermis Epidermis (b) Janice Carr/CDC Copyright © McGraw-Hill Education. Permission required for reproduction or display. E.g: Staphylococcus, Corynebacterium and Propionibacterium

10. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 10 Flora of the Gastrointestinal Tract GI tract is a long hollow tube, bounded by mucous membranes –Tube is exposed to the environment Variations in flora distribution are due to shifting conditions (pH, oxygen tension, anatomy) Oral cavity, large intestine, and rectum harbor appreciable flora The esophagus contains extremely light flora Stomach acid inhibits most microbes- Lactobacilli and Helicobacter Pharynx Esophagus Duodenum Large intestine Small intestine Rectum Anal canal Stomach Oral cavity Copyright © McGraw-Hill Education. Permission required for reproduction or display.

11. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 11 Flora of the Mouth Harbors the most abundant, diverse and unique flora of the body Numerous adaptive niches Cheek epithelium, gingiva, tongue, Mouth floor and tooth enamel Saliva is sterile until it enters the oral cavity. Bacterial count of saliva (5 x 10 9 cells per milliliter). Human bite is dangerous! Streptococcus sanguis, Salivarius, mutans, and mitis

12. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 12 Flora of the Large Intestine Has complex and profound interactions with host The cecum and colon harbors 10 8 -10 11 microbes per gram of feces (very abundant!- 30% of fecal volume) Intestinal environment favors anaerobic bacteria (Bacteriodes, Bifidobacterium, Fusobacterium and Clostridium, E. coli) Ferments waste materials, generates vitamins- B12, K, Pyridoxine, riboflavin and thiamine; acids- acetiC, butyric and propionic Intestinal bacteria contribute to intestinal odor by producing skatole –a chemical that gives feces their stench. They also produce flatus- intestinal gas (flatulence)

13. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 13 Flora of the Respiratory Tract Oral streptococci, first organisms to colonize Nasal entrance, nasal vestibule, anterior nasopharynx – S. aureus Mucous membranes of nasopharynx – Neisseria Tonsils and lower pharynx – Haemophilus Nasal vestibule Nasal entrance Nasal cavity Larynx Bronchus Bronchiole Internal naris Soft palate Nasopharynx Epiglottis Trachea Right lungLeft lung Sinuses Copyright © McGraw-Hill Education. Permission required for reproduction or display.

14. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 14 Flora of the Genitourinary Tract Sites that harbor microflora –Females – Vagina and outer opening of urethra –Males – Anterior ure Streptococci, staphylococci, corynebacteria, coliforms sometim Changes in physiology influence the composition of the normal flora –E.g. Vagina (estrogen, glycogen fermented by Lactobacilli pH) Anus Uterine tube Ovary Urinary bladder Urethra External reproductive organs Urinary bladder Penis Urethra Testis Rectum Vagina Anus Rectum Uterus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14

15. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 15 Maintenance of the Normal Resident Flora Normal flora is essential to the health of humans Flora create an environment that may prevent infections and can enhance host defenses Broad spectrum antibiotics, dietary changes, and disease may alter flora Probiotics – introducing pure cultures of known microbes back into the body through injection or inoculation

16. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Major Factors in the Development of an Infection 16

17. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 17 Major Factors in the Development of an Infection True pathogens – capable of causing disease in healthy persons with normal immune defenses –Influenza virus, plague bacillus, malarial protozoan Opportunistic pathogens – cause disease when the host’s defenses are compromised or when they grow in part of the body that is not natural to them –Pseudomonas sp & Candida albicans Severity of the disease depends on the virulence of the pathogen; characteristic or structure that contributes to the ability of a microbe to cause disease is a virulence factor.

18. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 18 Portals of entry – characteristic route a microbe follows to enter the tissues of the body Becoming Established Exogenous agents originate from source outside the body Endogenous agents already exist on or in the body (normal flora) Conjunctiva Respiratory tract Gastrointestinal tract Pregnancy and birth Urogenital tract Skin Copyright © McGraw-Hill Education. Permission required for reproduction or display.

19. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. –skin – nicks, abrasions, punctures, incisions –Gastrointestinal tract – food, drink, and other ingested materials –Respiratory tract – oral and nasal cavities –Urogenital tract – sexual, displaced organisms (STIs/STDs) –Transplacental- Syphilis spirochete Portals of Entry 19

20. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Pathogens That Infect during Pregnancy STORCH - Syphilis, Toxoplasmosis, Other diseases (hepatitis B, AIDS and chlamydia), Rubella, Cytomegalovirus and Herpes simplex virus Maternal blood pools within intervillous space (a)(b) Umbilical cord Umbilical arteries (fetal blood) Umbilical vein Bacterial cells Umbilical cord Maternal blood vessel Placenta Copyright © McGraw-Hill Education. Permission required for reproduction or display. 20 Could cause spontaneous abortion, congenital abnormalities, brain damage, prematurity and stillbirth

21. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Requirement for an Infectious Dose (ID) Minimum number of microbes required for infection to proceed Microbes with small IDs have greater virulence Lack of ID will not result in infection 21

22. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 22 Attaching to the Host Adhesion – microbes gain a stable foothold at the portal of entry; dependent on binding between specific molecules on host and pathogen –Fimbrae –Flagella –Glycocalyx –Cilia –Suckers –Hooks –Barbs –spikes

23. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Adhesion Properties of Microbes 23

24. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 24 Surviving Host Defenses Microbes invade host and evade host’s defenses using their virulence factors e.g: 3 types of virulence factor: 1. Antiphagocytic factors – used to avoid phagocytosis. Some can kill phagocytes - species of Staphylococcus and Streptococcus produce leukocidins which are toxic to white blood cells Slime layer or capsule – makes phagocytosis difficult e.g: Streptococcus pneumoniae, Salmonella typhi, Neisseria meningitidis Some have the ability to survive inside the phagocytes- provides a place to hide, grow and spread e.g Mycobacterium (c) Blocked phagocytic response

25. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 25 Entering Host Tissues Some pathogens produce a secretion system to insert specialized virulence proteins directly into the host cells e.g. Salmonella

26. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 26 Causing Disease Virulence factors – traits used to invade and establish themselves in the host, also determine the degree of tissue damage that occurs – severity of disease 2. Exoenzymes – dissolve extracellular barriers and penetrate through or between cells. E.g: mucinase, keratinase, collagenase and hyaluronidase, coagulase. 3. Toxigenicity – capacity to produce toxins at the site of multiplication. (a) Exoenzymes (b) Toxins Bacteria Epithelial cell Cell cement Exotoxins Toxins: chemical products of microbes, plants and some animals that has poisonous effects on other organisms

27. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Toxinoses: Any disease or lesion caused by the action of a toxin. Toxemias: toxinoses in which the toxin is spread by blood from the site of infection (e.g. tetanus) Intoxications (e.g. botulism) are caused by ingestion of toxins. 27

28. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 28 Bacterial Toxins: A Potent Source of Cellular Damage 2 Types of Bacterial Toxins according to origin: –Endotoxin – toxin that is not secreted but is released after the cell is damaged Composed of lipopolysaccharide (LPS), part of the outer membrane of gram-negative cell walls –Exotoxin – toxin molecule secreted by a living bacterial cell into the infected tissue Strong specificity for a target cell, deadly sometimes Hemolysins: disrupts cell membrane of RBC (HEMOLYSIS)- S. aureus, Streptococcus pyogenes A-B toxins (A-active, B-binding)

29. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 29 The Process of Infection and Disease 4 distinct stages of clinical infections: –Incubation period – time from initial contact with the infectious agent to the appearance of first symptoms; agent is multiplying but damage is insufficient to cause symptoms; –several hours in pneumonic plaque to several years in leprosy Height of infection Initial exposure to microbe Time Intensity of symptoms Incubation period Prodromal stage Period of invasion Convalescent period Copyright © McGraw-Hill Education. Permission required for reproduction or display.

30. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 30 The Process of Infection and Disease –Prodromal stage – vague feelings of discomfort; nonspecific complaints –Period of invasion – multiplies at high levels, becomes well-established; more specific signs and symptoms –Convalescent period – as person begins to respond to the infection, symptoms decline Height of infection Initial exposure to microbe Time Intensity of symptoms Incubation period Prodromal stage Period of invasion Convalescent period Copyright © McGraw-Hill Education. Permission required for reproduction or display.

31. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 31 Patterns of Infection Localized infection – microbes enter the body and remains confined to a specific tissue- boils, warts, fungal skin infection Systemic infection – infection spreads to several sites and tissue fluids usually in the bloodstream- viral diseases, bacterial: syphilis Focal infection – when infectious agent breaks loose from a local infection and is carried to other tissues- streptococcal pharyngitis gives rise to scarlet fever Localized infection (boil) Systemic infection (influenza) Focal infection (a)(b) (c) Copyright © McGraw-Hill Education. Permission required for reproduction or display.

32. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 32 Patterns of Infection Mixed infection – several microbes grow simultaneously at the infection site - polymicrobial Primary infection – initial infection Secondary infection – another infection by a different microbe Primary (urinary) infection Secondary (vaginal) infection Various microbes Mixed infection(d) (e) Copyright © McGraw-Hill Education. Permission required for reproduction or display.

33. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 33 Patterns of Infection Acute infection – comes on rapidly, with severe but short-lived effects Chronic infections – progress and persist over a long period of time

34. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Signs and Symptoms of Disease 34

35. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 35 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 –Lymphadenitis – swollen lymph nodes

36. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 36 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 –Septicemia – microorganisms are 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

37. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 37 Infections That Go Unnoticed Asymptomatic (subclinical) infections – although infected, the host doesn’t show any signs of disease Inapparent infection, so person doesn’t seek medical attention

38. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 38 Portals of Exit Pathogens depart by a specific avenue; greatly influences the dissemination of infection –Respiratory – mucus, sputum, nasal drainage, saliva –Skin scales –Fecal exit –Urogenital tract –Removal of blood Skin cells (open lesion) Coughing, sneezing Urine Insect bite Removal of blood Feces Copyright © McGraw-Hill Education. Permission required for reproduction or display.

39. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 39 Persistence of Microbes and Pathologic Conditions Apparent recovery of host does not always mean the microbe has been removed Latency – after the initial symptoms in certain chronic diseases, the microbe can periodically become active and produce a recurrent disease; person may or may not shed it during the latent stage Chronic carrier – person with a latent infection who sheds the infectious agent Sequelae – long-term or permanent damage to tissues or organs

40. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 40 Sources and Transmission of Microbes 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

41. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 41 Living Reservoirs Carrier – an individual who inconspicuously shelters a pathogen and spreads it to others; may or may not have experienced disease due to the microbe Asymptomatic carrier – shows no symptoms Passive carrier – contaminated healthcare provider picks up pathogens and transfers them to other patients (c)Transfer of infectious agent through contact Infectious agent Passive Copyright © McGraw-Hill Education. Permission required for reproduction or display.

42. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 42 Living Reservoirs Asymptomatic carrier – shows no symptoms –Incubation carriers – spread the infectious agent during the incubation period –Convalescent carriers – recuperating without symptoms –Chronic carrier – individual who shelters the infectious agent for a long period (a)(b) Stages of release during infection IncubationConvalescentChronicAsymptomatic Time Copyright © McGraw-Hill Education. Permission required for reproduction or display.

43. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 43 Animals as Reservoirs and Sources A live animal (other than human) that transmits an infectious agent from one host to another is called a vector Majority of vectors are arthropods – fleas, mosquitoes, flies, and ticks Some larger animals can also spread infection – mammals, birds, lower vertebrates Biological vectors – actively participate in a pathogen’s life cycle Mechanical vector – not necessary to the life cycle of an infectious agent and merely transports it without being infected

44. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 44 An infection indigenous to animals but naturally transmissible to humans is a zoonosis Humans don’t transmit the disease to others At least 150 zoonoses exist worldwide; make up 70% of all new emerging diseases worldwide Impossible to eradicate the disease without eradicating the animal reservoir Animals as Reservoirs and Sources

45. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Common Zoonotic Infections 45

46. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 46 Acquisition and Transmission of Infectious Agents Communicable disease – when an infected host can transmit the infectious agent to another host and establish infection in that 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

47. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 47 Patterns of Transmission Direct contact – physical contact or fine aerosol droplets Indirect contact – passes from infected host to intermediate conveyor and then to another host –Vehicle – inanimate material, food, water, biological products, fomites –Airborne – droplet nuclei, aerosols Copyright © McGraw-Hill Education. Permission required for reproduction or display. Marshall W. Jennison, Massachusetts Institute of Technology, 1940

48. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 48 How Communicable Infectious Diseases are Acquired

49. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 49 Nosocomial Infections Diseases that are acquired or developed during a hospital stay From surgical procedures, equipment, personnel, and exposure to drug- resistant microorganisms 2 to 4 million cases/year in U.S. with approximately 90,000 deaths Enterobacter spp. Enterococci Pseudomonas aeruginosa Staphylococcus aureus S. aureus Coagulase-negative staphylococci E. coli P. aeruginosa Coagulase-negative staphylococci Enterobacter spp. Enterococci P. aeruginosa S. aureus Acinetobacter spp. Enterobacter spp. Klebsiella pneumoniae Coagulase-negative staphylococci Candida spp. Enterobacter spp. Enterococci E. coli P. aeruginosa E. coli Candida spp. Other 12% Skin 8% Blood 6% Urinary tract 39% Lower respiratory tract 18% Surgical wounds 17% Copyright © McGraw-Hill Education. Permission required for reproduction or display.

50. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Preventing Nosocomial Infections 50

51. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 51 Universal Blood and Body Fluid Precautions Stringent measures to prevent the spread of nosocomial infections from patient to patient, from patient to worker, and from worker to patient – universal precautions Based on the assumption that all patient specimens could harbor infectious agents, so must be treated with the same degree of care

52. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 52 Epidemiology The study of the frequency and distribution of disease and health-related factors in human populations Surveillance – collecting, analyzing, and reporting data on rates of occurrence, mortality, morbidity and transmission of infections Reportable, notifiable diseases must be reported to authorities Centers for Disease Control and Prevention (CDC) in Atlanta, GA – principal government agency responsible for keeping track of infectious diseases nationwide

53. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 53 Frequency of Cases Prevalence – total number of existing cases with respect to the entire population usually represented by a percentage of the population Incidence – measures the number of new cases over a certain time period, as compared with the general healthy population Mortality rate – the total number of deaths in a population due to a certain disease Morbidity rate – number of people afflicted with a certain disease

54. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 54 Epidemiological Data from the CDC

55. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 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 Patterns of Infectious Disease Occurrence 55

56. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 56 Epidemic – when prevalence of a disease is increasing beyond what is expected Pandemic – epidemic across continents Patterns of Infectious Disease Occurrence

57. Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 57 Koch’s Postulates: Determining the causative agent of a disease 1.Find evidence of a particular microbe in every case of a disease 2.Isolate that microbe from an infected subject and cultivate it artificially in the laboratory 3.Inoculate a susceptible healthy subject with the laboratory isolate and observe the resultant disease 4.Reisolate the agent from this subject