1. Epidemiology and cycle of
Microbial Diseases
Dr. Chhaya Sawant
Shri C. B. Patel Research centre, Mumbai, India

2. Epidemiology means?
The science that evaluates patterns of disease
occurrence, frequency, determinants,
distribution, and control of health and disease in
a defined human population.
Epidemiology leads to the identification of causal
and preventive factors in human disease.

3. Epidemiology Defined
• ―The study of epidemics‖
• Greek roots
epi = upon (as in ―epidermis‖)
demos = the people (as in demography)
ology = ―to speak of‖, ―to study‖
• Modern definitions of epidemiology refer to
– distributions in populations (statistical)
– determinants
(pathophysiology, biology, chemistry, psychology)
– control of health problems
(biological, social, economic, political, administrativ
e, legal)

4. Epidemiology
• Also referred to as “Medical Ecology”. Why?
- Study of a disease in its natural environment.
Study of the clinical aspects and ecological
aspects of a given disease are important for the
public health measures to control the diseases
to be effective

5. Epidemiology
• Also referred to as “Medical Ecology”. Why?
- Study of a disease in its natural environment.
Study of the clinical aspects and ecological
aspects of a given disease are important for the
public health measures to control the diseases
to be effective

6. The Black Plague
• 1330’s, outbreak of plague in China.
• Spread to Italy and the rest of Europe.
• 25 million people died in just under five
years between 1347 and 1352 – 1/3 of the
population of Europe.
Influenza – 1918
Killed 50 to 100 million people worldwide, in
a single year.

7. The First Indications of Person-to-Person
Spread of an Infectious Disease
Edward Jenner (1749-1823) - developed a vaccine against smallpox using cow pox (160 years
before virus was identified)
In 1773 Charles White, an English surgeon and obstetrician, published his ―Treatise on the
Management of Pregnant and Lying-In Women‖ with Puerperal fever.
Puerperal fever is an acute febrile condition that can follow childbirth and is caused by
streptococcal infection of the uterus and/or adjacent regions.
In 1795 Alexander Gordon, a Scottish obstetrician, published his ―Treatise on the
Epidemic Puerperal Fever of Aberdeen‖ which demonstrated for the first time the
contagiousness of the disease.
In 1843 Oliver Wendell Holmes, a noted physician and anatomist in the United States, published a
paper entitled ―On the Contagiousness of Puerperal Fever‖.
Hungarian physician Ignaz Phillip Semmelweis (1847 and 1849 ), the first person to realize that a
pathogen could be transmitted from one person to another.
The pioneer of antisepsis in obstetrics – Saviors of Mothers. 32 years before causal agent was
discovered
John Snow (1813-1858) described the association between dirty water and cholera (44 years before
vibrio was identified

8. Brief History of Epidemiology
Classical Nutritional Epidemiology
–James Lind (1716-1794)
• conducted an experiment which showed that scurvy could
be treated and prevented with limes, lemons, and oranges
– ascorbic acid was discovered 175 years later
–Joseph Goldberger (1874-1927)
• identified that pellagra was not infectious but nutritional in
origin and could be prevented by increasing the amount of
animal products in the diet and substituting oatmeal for corn
grits
– niacin was discovered 10 years later

9. Objectives of Epidemiology in Emergencies
To identify the priority health problems in the affected
community
To determine the extent of disease existing within a
community
To identify the causes of disease and possible risk
factors
To determine the priority health interventions
To determine the extent of damage and capacity of
local infrastructure
To monitor health trends of the community
To evaluate the impact of health programs

10. Role of an Epidemiologist
• Traces the spread of a disease in a population
- To identify its origin
- Mode of Transmission
• With the help of Data Obtained from
- Clinical studies
- Disease reporting surveys
- Insurance questionnaires
- interviews with patients
This will help him define common factors that
constitute a disease.

11. Important Terms
• Attack rate : The proportional number of cases developing in the population that was
exposed to the infectious agent
• Communicable disease : An infectious disease that can be transmitted from one host
to another
• Non-communicable disease : A disease that is not transmitted from one host to
another
• Herd immunity : A phenomenon that occurs when a critical concentration of immune
hosts prevents the spread of an infectious agent
• Incidence : The number of new cases of a disease in a population at risk during a
specified period of time
• Index case : The first identified case of a disease in an outbreak or epidemic
• Outbreak : A cluster of cases occurring during a brief time interval and affecting a
specific population; an outbreak may herald the onset of an epidemic
• Portal of entry : Surface or orifice through which a disease-causing agent enters the
body
• Portal of exit : Surface or orifice from which a disease-causing agent exits and
disseminates
• Prevalence : The total number of cases in a given population at risk at any point in
time
• Reservoir : The natural habitat of a disease-causing organism

12. Infectious disease: the unique factor
Historical approach :
Infectious diseases can be spread from human to human
(or animal to human)
Modern Approach :
Alcohol and drug abuse, cancer, mental conditions, acts of
violence and exposure to lead paint.

13. Cycle of Microbial Disease
Person to person
Reservoir transmission
Portal of exit Susceptible Host
Portal of entry
Agent
Mode of transmission

14. Chain of transmission
Reservoir
Human
Person with symptomatic illness
Carriers:
Asymptomatic
Incubating
Convalescent
Chronic
Animal: zoonosis
Environmental: soil, plant, water

15. Chain of transmission
Portal of exit
Human/animal
Respiratory tract
Genito-Urinary tract
Faeces
Saliva
Skin (exanthema, cuts, needles, blood-sucking arthropods)
Conjunctival secretions
Placenta
Environmental
Cooling towers

16. Chain of transmission
Mode of Transmission
Direct
Direct contact
Secretions,
Blood,
Faeces/urine
Droplet spread
Indirect
Food/water
Aerosol
Animal vectors
Fomites
Medical devices
and treatments

17. Transmission
• Transmission of a disease from its reservoir to the
next susceptible host.
• Through contact, ingestion of food or water, or via a
living agent such as an insect is called horizontal
transmission.
• Vertical transmission, the transfer of a pathogen
from a pregnant woman to the fetus, or from a mother
to her infant during childbirth.
e.g. Congenital syphilis, Group B streptococci, can
infect the newborn as it passes through the birth canal

18. Chain of transmission
Portal of entry
Human/animal
Respiratory tract
Genito-Urinary tract
Faeces
Saliva
Skin
(exanthema, cuts, needles, blood-
sucking arthropods)
Conjunctival secretions
Placenta

19. Frequency and distribution based
Classification
• Sporadic level: occasional cases occurring at irregular
intervals
• Endemic level: persistent occurrence with a low to moderate
level
• Hyper Endemic level: persistently high level of occurrence
• Epidemic : occurrence clearly in excess of the expected level
for a given time period
• Pandemic: epidemic spread over several countries or
continents, affecting a large number of people

20. A Graph Illustrating Three Epidemics
The solid blue line indicates the expected number of endemic cases.
The connected red dots indicate the actual number of cases.
Epidemics (marked by brackets) are sharp increases in the number of cases of a
disease above that which is normally expected (solid line).

21. Disease Progression
• Infection
• Incubation period
• Prodromal stage
• Period of illness
• Convalescence

22. Factors that Influence the Epidemiology
of Disease
• The Dose
• The Incubation Period
• Population Characteristics
- Immunity to the pathogen.
- General health.
- Age.
- Gender
- Religious and cultural practices.
- Genetic background.

23. Measuring Frequency
• Three important statistical measures of disease
frequency
– morbidity rate
– prevalence rate
– mortality rate

24. Morbidity rate :
Measures the number of individuals that become ill due to a
specific disease within a susceptible population during a
specific time interval.
Morbidity rate =
Number of new cases during a specific time
Total number of individuals in population
• E.g. 700 new cases of influenza per 100,000 individuals is
0.7%.
Prevalence Rate
The total number of individuals infected in a population at
any one time no matter when the disease began

25. • Mortality rate
Number of deaths from a disease per number of cases of the
disease
• Mortality rate =
Number of deaths due to given disease
size of total population with disease
If 500 people in a town of 100,000 become infected with HIV and 100
die, the mortality rate is…

26. Recognition of an Infectious Disease in a Population
1. Generation of morbidity data from case reports
2. Collection of mortality data from death certificates
3. Investigation of actual cases
4. Collection of data from reported epidemics
5. Field investigation of epidemics
6. Review of laboratory results: surveys of a population for antibodies against
the agent and specific microbial serotypes, skin tests, cultures, stool
analyses, etc.
7. Population surveys using valid statistical sampling to determine who has the
disease
8. Use of animal and vector disease data
9. Collection of information on the use of specific biologics—
antibiotics, antitoxins, vaccines, and other prophylactic measures
10. Use of demographic data on population characteristics such as human
movements during a specific time of the year
11. Use of remote sensing and geographic information systems

27. • Recognition of an Epidemic
• Two types of epidemics
– common source epidemic
– propagated epidemic
• Common source epidemic
• Reaches a peak within a short period of time -1 to 2 weeks)
• A moderately rapid decline in the number of infected patients
• A single common contaminated source - food (food poisoning)
or water (Legionnaires’ disease).
• Propagated epidemic (Host to Host)
• A relatively slow and prolonged rise and then a gradual decline
in the number of individuals infected.
• A single infected individual into a susceptible population.
• E.g. Increase in mumps or chickenpox cases, Spread of HIV
infection.

28. Seasonal Cycles of diseases
• The season of the year in which the epidemic
occurs may also be significant.
• Respiratory diseases including Influenza, Respiratory Syncytial
Virus infections, and the common cold are more easily
transmitted in crowded indoor conditions during the winter.
• Conversely, vector- and food-borne diseases are more often
transmitted in warm weather when people are more likely to be
exposed to mosquitoes and ticks, or eating picnic food that has
not been stored properly.

29. Seasonal Cycles of diseases
Seasonal Occurrence of Seasonal Occurrence of
Respiratory Infections Caused by Gastrointestinal Diseases
Respiratory Syncytial Virus

30. Herd Immunity.
Resistance of a population to
infection and to spread of an
infectious organism because of
the immunity of a large
percentage of the population
The kinetics of the spread of an
infectious disease and the effect
of increasing the number of
immune individuals in the
population in limiting the disease.
On day 1, a single infected
individual enters the population.
The incubation period is 1 day,
and recovery occurs in 2 days.
The number of susceptible
individuals is the total population
on day 1.

31. Diagrammatic representation of the spread of an imaginary propagated epidemic.
The lower curve represents the number of cases and the upper curve the number of
susceptible individuals. Notice the coincidence of the peak of the epidemic wave
with the threshold density of susceptible individuals.

32. The number of infected and recovered are illustrated in the two graphs.

33. Herd immunity
• Resistance of a population to infection and to spread
of an infectious organism because of the immunity of a
large percentage of the population
• Level can be altered by introduction of new
susceptible individuals into population
• Level can be altered by changes in pathogen
– antigenic shift – major change in antigenic character of
pathogen (recombination in birds, pigs totally new antigen
types)
– antigenic drift – smaller antigenic changes (point-mutational
changes)

34. Virulence and the Mode of Transmission
Evidence suggests correlation between mode
of transmission and degree of virulence
– direct contact less virulent
– vector-borne highly virulent in human host;
relatively benign in vector
– greater ability to survive outside host more
virulent

35. The host community:
• Infection --- immunity --- coexistence
• Host-pathogen
• co evolution: the case of the rabbit-myxoma virus
– Transmitted by mosquitoes
– Rabbit mortality:
susceptibility of new born rabbits to a moderately virulent strain of
the virus
• – Virus virulence: infection of lab rabbits with wild virus
1950 - 1980: Pre introduction levels of rabbit
infestation Introduction of a second virus
(RHDV; host-to-host transmission; instant
killing) --- co evolution.

36. Emerging and Reemerging Infectious
Diseases and Pathogens
By the 1990s, the idea that infectious diseases
no longer posed a serious threat to human
health was obsolete.
It is now clear that globally, humans will
continually be faced with both new infectious
diseases and the reemergence of older
diseases once thought to be conquered
(e.g., tuberculosis, dengue hemorrhagic
fever, yellow fever)

37. Emerging and Reemerging Infectious
Diseases and Pathogens
Infectious Disease Mortality in the United States Decreased Greatly during Most of
the Twentieth Century.
The insert is an enlargement of the right-hand portion of the graph and shows that the
death rate from infectious diseases increased between 1980 and 1994.

38. Systematic epidemiology
The increased importance of emerging and
reemerging infectious diseases has stimulated the
establishment of a field called
systematic epidemiology
Which focuses on the ecological and social factors that
influence the development of these diseases.

39. Factors characteristic of the modern world favoring the
development and spread of these microorganisms and their
diseases
1. Unprecedented worldwide population growth, population shifts
(demographics), and urbanization
2. Increased international travel
3. Increased worldwide transport (commerce), migration, and relocation of
animals and food products
4. Changes in food processing, handling, and agricultural practices
5. Changes in human behavior, technology, and industry
6. Human encroachment on wilderness habitats that are reservoirs for insects
and animals that harbor infectious agents
7. Microbial evolution (e.g., selection pressure) and the development of
resistance to antibiotics and other antimicrobial drugs (e.g., penicillin-
resistant Streptococcus pneumoniae, methicillin-resistant Staphylococcus
aureus, and vancomycin-resistant enterococci)

40. Factors characteristic of the modern world favoring the
development and spread of these microorganisms and their
diseases
8. Changes in ecology and climate
9. Modern medicine (e.g., immunosuppression)
10. Inadequacy of public infrastructure and vaccination programs
11. Social unrest and civil wars
12. The possibility of bioterrorism
13. Virulence-enhancing mechanisms of pathogens (e.g., the mobile
genetic elements—bacteriophages, plasmids, transposons)

41. Some Examples of Emerging and Reemerging Infectious Diseases.
Although diseases such as HIV are indicated in only one or two significant locations,
they are very widespread and a threat in many regions.

42. Public Health System
The public health system comprises of a
network of clinical microbiologists, nurses,
physicians, epidemiologists, and infection control
personnel who supply epidemiological
information to a network of local, state, national,
and international organizations.

43. The Role of the Public Health System
• Control of Epidemics (Three Types)
• First Type : Directed toward reducing or
eliminating the source or reservoir of infection
1. Quarantine and isolation of cases and/or carriers
2. Destruction of an animal reservoir of infection
3. Treatment of sewage to reduce water contamination
4. Therapy that reduces or eliminates infectivity of the
individual

44. Control of Epidemics (Three Types)
• The second type : designed to break the
connection between the source of the infection
and susceptible individuals. (general sanitation
measures - Transmission)
1. Chlorination of water supplies
2. Pasteurization of milk
3. Supervision and inspection of food and food handlers
4. Destruction of vectors by spraying with insecticides

45. Control of Epidemics (Three Types)
• The Third type : reduces the number of
susceptible individuals and raises the general
level of herd immunity by immunization.
1. Passive immunization to give a temporary
immunity following exposure to a pathogen or when
a disease threatens to take an epidemic form
2. Active immunization to protect the individual from the
pathogen and the host population from the epidemic
The most important is Surveillance – Observation,
recognition and reporting of Diseases as they occur.

46. Precautions to be taken by individuals to prevent
travel-related infectious diseases.
1. If one is traveling to an area where malaria is
endemic, weekly prophylaxis before entering the area and
after leaving the area is recommended.
2. Travelers should recall the benefits of abstinence or protective
sexual practices, especially the use of condoms. Hepatitis B
vaccine should be administered if it is indicated.
3. Travelers should avoid uncooked food, non bottled water and
beverages, and unpasteurized dairy products. Use bottled
water for drinking, making ice cubes, and brushing teeth.
4. Wash hands with soap and water frequently, especially before
each meal.

47. Precautions to be taken by individuals to prevent
travel-related infectious diseases.
5. To prevent respiratory infections, avoid excessive outdoor
activities in areas of heavy air pollution during hot or humid
parts of the day. Consider tuberculin skin testing before and
after travel.
6. Minimize skin exposure and use repellents to prevent
arthropod-borne illnesses (e.g., malaria, dengue, yellow fever,
Japanese encephalitis).
7. Avoid skin-perforating procedures (e.g., acupuncture, body
piercing, tattooing, venipuncture, sharing of razors).
8. Do not pet or feed animals, especially dogs and monkeys.
9. Avoid swimming or wading in non-chlorinated freshwater.

48. Nosocomial Infections
• Nosocomial diseases - caused by bacteria, most of which are
noninvasive and part of the normal microbiota;
• Viruses, protozoa, and fungi are rarely involved.
• Source of Hospital Infection
• Endogenous sources - Patient’s own microbiota.
• Exogenous sources are other than the patient’s own microbiota
.
• In either case the pathogen colonizing the patient may
subsequently cause a nosocomial disease.

50. The Hospital Epidemiologist
• The services provided by the hospital epidemiologist
should include
1. Research in infection control
2. Evaluation of disinfectants, rapid test systems, and other products
3. Efforts to encourage appropriate legislation related to infection control,
particularly at the state level
4. Efforts to contain hospital operating costs, especially those related to fixed
expenses such as the DRGs (diagnosis related groups)
5. Surveillance and comparison of endemic and epidemic infection frequencies
6. Direct participation in a variety of hospital activities relating to infection
control and maintenance of employee health
7. Education of hospital personnel in communicable disease control and
disinfection and sterilization procedures
8. Establishment and maintenance of a system for identifying, reporting,
investigating, and controlling infections and communicable diseases of
patients and hospital personnel
9. Maintenance of a log of incidents related to infections and communicable
diseases
10. Monitoring trends in the antimicrobial drug resistance of infectious agents

51. The Emerging Threat of Bioterrorism
• The Centers for Disease Control and
Prevention recently defined bioterrorism as
―The intentional or threatened use of viruses, bacteria,
fungi, or toxins from living organisms to produce death
or disease in humans, animals, and plants.‖
• a few kilograms of anthrax can kill as many people as
a Hiroshima-size nuclear bomb
The goal of bioterrorism is to produce fear in
the population with subsequent disruption of
society

52. History of Bioterrorism
• Biological warfare (BW) employed as far back as 6th century
BC.
• Examples of past BW:
14th Century: Mongols catapulted corpses with bubonic plague over walls
into Crimea.
15th Century: Pizarro presented native South Americans with smallpox-
contaminated clothing.
1940: Japan’s ―Unit 731‖ dropped plague-infected fleas over Manchuria &
China.
1984 : Rajneeshee Cult contaminated restaurant salad bars with Salmonella
typhimurium.
1995: Aum Shinrikyo cult attempted unsuccessfully to disperse BW agents
in aerosol form; sarin gas attack in Tokyo.
2001: Anthrax-contaminated letters to U.S. media and government offices.

53. • In 1998, the U.S. government launched the first
national effort to create a biological weapons defense.
The initiatives include
• The first ever procurement of specialized vaccines and
medicines for a national civilian protection stockpile;
• investment of more time and money in genome sequencing,
new vaccine research, and new therapeutic research;
• development of improved detection and diagnostic systems;
• preparation of clinical microbiologists and the clinical
microbiology laboratory as members of the ―first responder‖
team, which is to respond in a timely manner to acts of
bioterrorism.

54. Preventions: International
treaties, strategic
preparedness (vaccines), make this
world a better place for everyone
• Awareness
• Laboratory Preparedness
• Plan in place
• Individual & collective protection
• Detection & characterization
• Emergency response
• Measures to Protect the Public’s
Health and Safety
• Treatment
• Safe practices

55. Future Challenges to Public Health
• Emergence of new infectious diseases and re-emergence of
old ones.
• Inequities in health care.
• Threat of large scale terrorist attacks using nuclear, biological
and chemical agents.
• Chronic diseases – coronary heart disease, obesity, diabetes,
cancer.
• Continued political instability.
• International coordination of public health efforts.
• Ethics.
• Ignorance.

56. Types of Epidemiological Studies Undertaken
• Descriptive Studies – The Person, The Place, The Time,
• Analytical Studies
- Cross-Sectional Studies : A cross-sectional study surveys a range of
people to determine the prevalence of any of a number of characteristics
including disease, risk factors associated with disease, or previous exposure
to a disease-causing agent.
- Retrospective Studies : A retrospective study is done following a
disease outbreak. This type of study compares the actions and events
surrounding clinical cases (individuals who developed the disease) against
appropriate controls (those who remained healthy).
- Prospective Studies : A prospective study is one that looks ahead to
see if the risk factors identified by the retrospective study predict a tendency
to develop the disease. Cohort groups, which are study groups that have a
known exposure to the risk factor, are selected and then followed over time.
• Experimental Studies

57. Investigation of a GI illness
outbreak
http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

58. Investigation of a GI illness outbreak
http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

59. Investigation of a GI illness outbreak
If you were to administer a questionnaire to the church
supper participants, what information would you collect?
1. What did you eat?
2. How much did you eat?
3. How long after you ate did you begin to feel sick?
4. How long did it last?
5. Did anyone at the supper show illness prior to the supper?
6. What did you drink?
7. How was the food prepared/ stored?
http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

60. Investigation of a GI illness outbreak

61. Investigation of a GI illness outbreak
http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

62. Investigation of a GI illness outbreak
Without having clinical isolates from the infected
individuals for examination, how might you tentatively
identify the causative agent?
1. Temperature of storage vs organism?
2. Food vs organism?
3. Homemade vs commercial?
4. Symptoms
5. Culture ice cream.
It the outbreak has already ended, what is the value of
working up the case?