1. Epidemiology Kept Simple
Chapter 3
The Infectious Disease Process
Jul-19

2. Reasons to Study Infx D. Epi
Infectious disease control, including
emerging and reemerging agents
Bioterrorism
To illustrate general principals of epidemiology (“One Epi”)
Jul-19

3. What is Infection?
Infection ≡ biologic agent living and replicating within host
Contamination ≡ agent living on exterior surface of host
Silent Infection ≡ infection without disease (commensal)
Infectious disease ≡ infection accompanies by pathology Infection ≠ disease!
Pathos
Jul-19

4. Components of the Infx Disease Process
Agent
Reservoir
Portals of entry and exit
Mode of transmission
Immunity
Jul-19

5. 1. Agents Helminths (parasitic worms)
Fungi & yeast (parasitic plants, lack chlorophyll)
Protozoa (eucaryotes; complex life cycles)
Bacteria (independent reproduction)
Rickettsia (intracellular agents; require Ixodes tick carrier)
Viruses (submicroscopic; incapable of multiplication outside of host)
Prions (infectious proteins)
Jul-19

6. 2. Reservoirs Reservoir ≡ where the agent multiplies and perpetuates
Types of reservoirs
Symptomatic cases
Carriers
Animals & insects
Inanimate objects
Jul-19

7. Cases Reservoirs Some (not all) cases are contagious
Examples of agents with case reservoirs
Influenza
Measles
Smallpox
Sexually transmitted diseases
Jul-19

8. Carrier Reservoirs
Carrier ≡ contagious individual without discernable signs
Types of carriers
Inapparent throughout
Incubatory
Convalescent
Jul-19

9. Animals Reservoirs (Zoonoses)
Human disease with an animal reservoir is called a zoonosis.
Types of zoonoses
Direct zoonosis:  vertebrate animal  human e.g., rabies
Cyclozoonoses:  species #1 species #2  human  species #1 (e.g., Echinococcus tapeworm)
Jul-19

10. Types of Zoonoses (cont.)
Metazoonoses
 vertebrate animal  invertebrate  human e.g., malaria
Saprozoonoses
vertebrate animal  inanimate object  human
e.g. (coccidiomycosis) Valley fever
Jul-19

11. Inanimate Reservoirs
Water
Food
Soil
Other
Jul-19

12. Portals of Entry & Exit Respiratory Conjunctiva Urogenital
Gastrointestinal
Skin
Placenta
Etc.
Jul-19

13. Transmission by Contact
Direct (host  host)
Indirect (host  secretion  host)
Droplet (airborne, short distance)
Nuclei (airborne, suspended)
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14. Transmission via Intermediaries
Vector (living)
Vehicles (inanimate)
Vectors types: mechanical, developmental, propagative, cyclopropagative.
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15. Transmission Dynamics
Common source
Serial transmission
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16. Transmission: Cycle in Nature
Each agent has its unique cycle in nature
Example: the blood worm (Shistosoma sp.).
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17. Immunity
Immunity ≡ all factors that alter likelihood and severity of infection after host is exposed
Types of immunity (figure)
Jul-19

18. Innate Immunity Physical barriers: skin, cilia, mucosal, sheaths
Chemical barriers: acidity, enzymatic, etc.
Non-specific cellular & physiologic responses: phages, polymorphs, inflammation
Jul-19

19. Acquired Immunity Cellular (immunocytes) Lymphocytes Granulocytes
Non-cellular (humoral)
Antibodies
Cytokines
Jul-19

20. Interaction of Innate & Acquired Immunity
Jul-19

21. Immunization Immunization ≡ the act of acquiring immunity
Active immunization ≡ host response to exposure (Natural exposure or artificial vaccination)
Passive immunization ≡ receipt of immunity products from others (Therapeutic e.g., anti-serums or maternal (transplacental, colostrum)
Jul-19

22. Types of Vaccines
Killed vaccine: killed agent, not capable of self-replication
Modified live vaccine: attenuated version of agent capable of replication
Toxoid: denatured toxin (no agent)
Jul-19

23. Herd Immunity Non-susceptible individuals represented by dark circles.
You do not need to vaccinate the entire herd to achieve control
Jul-19

24. Herd Immunity Animation
Jul-19