1. Principles of Disease and Epidemiology
Chapter 14
Principles of Disease and Epidemiology

2. Principles of Disease and Epidemiology
Pathology Study of disease
Etiology Study of the cause of a disease
Pathogenesis Development of disease
Infection Colonization of the body by pathogens
Disease An abnormal state in which the body is not functionally normally

3. Normal Microbiota and the Host
Transient microbiota may be present for days, weeks, or months
Normal microbiota permanently colonize the host
Symbiosis (تعايش) is the relationship between normal microbiota and the host

4. Normal Microbiota and the Host:
In commensalism, one organism is benefited and the other is unaffected.
In mutualism, both organisms benefit.
In parasitism, one organism is benefited at the expense of the other.
Some normal microbiota are opportunistic pathogens.

5. Normal Microbiota and the Host:
Locations of normal microbiota on and in the human body
Figure 14.2

6. Normal Microbiota and the Host:
Microbial antagonism is competition between microbes.
Normal microbiota protect the host by:
occupying niches that pathogens might occupy
producing acids
producing bacteriocins
Probiotics are live microbes applied to or ingested into the body, intended to exert a beneficial effect.

7. Koch’s Postulates
Koch's Postulates are used to prove the cause of an infectious disease.
Figure

8. Koch’s Postulates
Koch's Postulates are used to prove the cause of an infectious disease.
Figure

9. They are summarized as follows:
1. The same pathogen must be present in every case of the
disease.
2. The pathogen must be isolated from the diseased host and grown in pure culture.
3. The pathogen from the pure culture must cause the disease when it is inoculated into a healthy, susceptible laboratory animal.
4. The pathogen must be isolated from the inoculated animal and must be shown to be the original organism.

10. Exceptions to Koch's Postulates
Some microbes have unique culture requirements:
Treponema pallidum is known to cause syphilis (الزهري), but virulent strains have never been cultured on artificial media.
The causative agent of leprosy (الجذام), Mycobacterium leprae, has also never been grown on artificial media.
Many rickettsial and viral pathogens cannot be cultured on artificial media because they multiply only within cells.

11. Classifying Infectious Diseases
Symptom A change in body function that is felt by a patient as a result of disease
Sign A change in a body that can be measured or observed as a result of disease.
Syndrome A specific group of signs and symptoms that accompany a disease.

12. Classifying Infectious Diseases
Communicable disease A disease that is easily spread from one host to another (either directly or indirectly) Chickenpox, measles, genital herpes, typhoid fever
Contagious disease A disease that is easily spread from one host to another.(directly) Chickenpox, measles
Noncommunicable disease A disease that is not transmitted from one host to another.(tetanus: Clostridium tetani)

13. Occurrence of Disease
Incidence Fraction of a population that contracts a disease during a specific time.
Prevalence Fraction of a population having a specific disease at a given time.
Sporadic disease Disease that occurs occasionally in a population.
Endemic disease Disease constantly present in a population.(common cold)
Epidemic disease Disease acquired by many hosts in a given area in a short time.(influenza)
Pandemic disease Worldwide epidemic.(influenza, AIDS)
Herd immunity Immunity in most of a population.

14. Severity or Duration of a Disease
Acute disease Symptoms develop rapidly but lasts only a short time; influenza
Chronic disease Disease develops slowly (tuberculosis, Hepatitis B)
Subacute disease Symptoms between acute and chronic (panencephalitis)
Latent disease Disease with a period of no symptoms when the patient is inactive(shingles)

15. Extent of Host Involvement
Local infection Pathogens limited to a small area of the body (boils and abscesses)
Systemic infection An infection throughout the body (measles)
Focal infection Systemic infection that began as a local infection
Bacteremia Bacteria in the blood
Septicemia Growth of bacteria in the blood

16. Extent of Host Involvement
Toxemia Toxins in the blood
Viremia Viruses in the blood
Primary infection Acute infection that causes the initial illness
Secondary infection Opportunistic infection after a primary (predisposing) infection
(streptococcal bronchopneumonia following influenza)
Subclinical disease No noticeable signs or symptoms (inapparent infection).. Poliovirus and hepatitis A virus

17. Predisposing Factors Make the body more susceptible to disease
Short urethra in females
Inherited traits such as the sickle-cell gene (Malaria)
Climate and weather
Fatigue
Age
Lifestyle
Chemotherapy

18. The Stages of a Disease

19. Reservoirs of Infection
Reservoirs of infection are continual sources of infection.
Human — AIDS, gonorrhea
Carriers may have inapparent infections or latent diseases
Animal — Rabies, Lyme disease
Some zoonoses may be transmitted to humans
Nonliving — Botulism, tetanus
Soil

20. Transmission of Disease
Contact
Direct Requires close association between infected and susceptible host
Indirect Spread by fomites
Droplet Transmission via airborne droplets
Bacterial :Tuberculosis, Pneumococcal infection, Meningococcal infection, Staphylococcus aureus infection. Viral: Chickenpox, Measles, Influenza, Fungal: Aspergillus, Cryptococcus neoformance

21. Transmission of Disease
Figure 14.6a & 8

22. Transmission of Disease
Vehicle Transmission by an inanimate reservoir (food, water)
Vectors Arthropods, especially fleas, ticks, and mosquitoes
Mechanical Arthropod carries pathogen on feet
Biological Pathogen reproduces in vector

23. Transmission of Disease
Figure 14.6b, c

24. Nosocomial (Hospital-Acquired) Infections
Are acquired as a result of a hospital stay
5-15% of all hospital patients acquire nosocomial infections
Figure 14.7, 9

25. Relative frequency of nosocomial infections
Figure 14.10

26. Common Causes of Nosocomial Infections
Percentage of nosocomial infections
Percentage resistant to antibiotics
Gram + cocci
34%
28%-87%
Gram – rods
32%
3-34%
Clostridium difficile
17%
Fungi
10%

27. Infection by hospital equipment
Infection from surgical instruments is now rare. But instruments can convey infection from one patient to another. Following infections have been reported to be transmitted from instruments:
Rhizopus
Mucor
Clostridium
Salmonellosis (through rectal thermometers )
Hepatitis
Mycobacterium tuberculosis

28. Infection by inoculation
The infections through inoculation can be transmitted in the following ways:
Through blood and blood products.
Accidental injury from sharp contaminated insruments.
Through unsterile syringes and needles.

29. Infection by inoculation
Various organisms and diseases which have been reported to be transmitted through this mode include:
Hepatitis B,C,D.
Brucellosis
Salmoellosis
Treponemes
Toxoplasmosis

30. Emerging Infectious Diseases
Diseases that are new, increasing in incidence, or showing a potential to increase in the near future.
Contributing factors:
Evolution of new strains
V. cholerae O139
Inappropriate use of antibiotics and pesticides
Antibiotic resistant strains
Changes in weather patterns
Hantavirus

31. Emerging Infectious Diseases
Contributing factors:
Modern transportation
West Nile virus
Ecological disaster, war, expanding human settlement
Coccidioidomycosis
Animal control measures
Lyme disease
Public Health failure
Diphtheria