1. Principles of Disease and Epidemiology

2. Pathology Branch of medicine to study disease
Cause or etiology of disease
Manner which disease develops
Structural and functional changes by disease and effects on host

3. Infection and Disease Exposure – left unprotected to contact with MO
Infection - invasion or colonization of body by pathogenic MO
Disease - any change from normal state of health or abnormal state, in which body not properly adjusted or not capable of carrying on normal function

4. Infection and Disease Infection not necessarily indicate disease
Normal flora - may colonize or infect specific areas of body without causing disease; may however cause disease given right circumstances
Person infected with MO (i.e., HIV) but show no symptoms of disease

5. Normal Flora: Opportunist
E. coli NF in stool, but #1 cause of urinary tract infection
Viridans group Streptococcus NF in oropharyngeal area, but cause subacute bacterial endocarditis when gains entrance into bloodstream
Considered opportunistic pathogen

6. Normal Flora: Carriers
In addition to usual NF, some persons colonized by MO generally considered pathogenic, but without disease
These individuals are carriers and transmit pathogenic MO to others who develop disease
Neisseria meningitidis – URT
Salmonella typhi – GI tract, gall bladder (reservoir site)

7. Types of Infectious Disease
Infectious disease – by disease producing MO
Congenital disease – present at birth and result of condition in utero (maternal infection, use of drug or alcohol, etc.); may result in birth defects
Iatrogenic disease – caused by health care personnel during delivery of care; could be due to use of contaminated equipment, administration of drug, etc.
Nosocomical disease - hospital acquired disease
Idiopathic disease – undetermined cause

8. Nosocomial Infections
Hospitals provide perfect environment for MOs, in part because patient with weakened condition and more susceptible to infection and disease
Up to 15% of hospitalized patients get a nosocomial infection
Most nosocomial infections caused by opportunistic pathogens typically considered NF

9. Types of Nosocomial Infections
Septicemia

10. Koch’s Postulates: Etiology of Infectious Disease
1 – Pathogen present in every case of disease
2 - Pathogen isolated from diseased host and grown in pure culture
3 - Pathogen must cause same disease when inoculated into healthy, susceptible host
4 - Pathogen must again be isolated from inoculated animal, shown to be same as originally isolated MO
Not possible to identify etiology of all infectious diseases following Koch’s postulates:
MO not isolated on culture media
MO not inoculated into healthy human host to cause disease

11. Epidemiology
Study of when and where diseases occur and how transmitted in a population
Also concerned with methods of controlling and preventing a disease; drugs, vaccines, and reservoirs
State and Federal Public Health Department
The Centers for Disease Control and Prevention, (CDC), Atlanta, Georgia
The World Health Organization (WHO), within the United Nations

12. Spread of Disease: Reservoirs
For a disease to perpetuate, must be a source of infection called a reservoir
Humans – Carriers or sick individuals during incubation, acute, or convalescent phases (when transmitted depends on disease)
Animals – diseases found in animals called zoonosis
Non-living entities - such as soil or water

13. Spread of Disease: Transmission
Causative agents transmitted from the reservoir of infection to a susceptible host via four main routes:
1. Contact
2. Common Vehicle
3. Airborne
4. Vector

14. 1. Contact Transmission
Direct contact – spread from person to person via touching, kissing, or sexual intercourse
Indirect contact – to a susceptible host by means of non-living object called fomite (i.e., contaminated needles)
Droplet transmission – spread by droplets in saliva and mucous; discharged by sneezing, coughing, laughing, or talking. The droplets travel only short distance (< 1 meter) and not considered airborne

15. 2. Common Vehicle Transmission
Refers to transmission of disease causing agents by a common inanimate reservoir to a large number of individuals
Food
Water
Blood
Drugs

16. 3. Airborne Transmission
Refers to spread of agents of infection by droplet nuclei or dust
In this instance the particles travel > 1 meter from the reservoir to the host
i.e., air condition unit

17. 4. Vector Transmission
An animal that carries pathogen from one host to another
Arthropods are most common type
Transmit disease either by:
Mechanical – pathogen on feet or other body parts to food ingested by humans
Biological – arthropod bites infected host, acquires the pathogen, MO reproduce inside the vector, pathogen transmitted to new host via salivary glands or feces

18. Infectious Disease: How Spread
Communicable disease – spreads from one host to another either directly or indirectly
Noncommunicable disease – doesn’t spread from host to host but caused by MO:
Normally inhabits body and occasionally causes disease
Resides outside the body and produces disease when introduced into the body
Contagious disease – a disease that spreads easily from one person to another

19. Infectious Disease: Severity/Duration
Acute disease – develops rapidly, lasts relatively short time i.e. “cold”
Chronic disease – develops slowly, body’s reaction usually less severe, but disease is continuous or recurrent for long period i.e. TB
Subacute disease – intermediate between acute and chronic i.e. “infectious” hepatitis
Latent disease – MO remains inactive for long period, then becomes active and produce symptoms i.e. chickenpox/shingles

20. Infectious Disease: Frequency of Occurrence
Sporadic – occurs occasionally i.e. measles in USA
Endemic – constantly present in population i.e. TB inner city LA
Epidemic – many people in a given area acquire disease in short period of time i.e. pertussis in USA 2010
Pandemic – worldwide epidemic i.e. flu

21. Infectious Disease: Health of Body
Primary infection – acute infection causes initial illness
Secondary infection – by opportunistic pathogen after primary infection has weakened the body’s defenses
Inapparent or subclinical infection – doesn’t cause any noticeable illness (i.e., healthy carriers)

22. Infectious Disease: Signals
Symptoms – subjective changes in body function such as pain or malaise (not measurable)
Signs – objective changes that can be observed and measured; such as fever, swelling, or rash
Syndrome – a group of symptoms and signs that always accompany a particular disease

23. Infectious Disease: Stages
Period of incubation – time between acquiring infection and appearance of first symptoms/signs
Prodromal period – first signs/symptoms appear
Period of illness – disease most acute, overt signs/symptoms occur (increase or decrease in WBCs may occur here)

24. Infectious Disease: Stages
Period of decline – where signs/symptoms subside
If the decline occurs quickly, it is said to occur by crisis
If the decline occurs over longer period of time, it is said to occur by lysis
Period of convalescence - where person regains strength and body returns to pre-diseased state

25. Class Assignment Textbook Reading: Key Terms
Chapter 2 Host- Pathogen Interaction
B. Pathogenesis of Infection
Routes of Transmission
Key Terms
Learning Assessment Questions
Review, Review, Review!