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

2. Reasons to Study Infx D. Epi
Infectious disease control, including emerging agents and bioterrorism
To illustrate general principals of epidemiology
Apr-19

3. What is Infection?
Infection ≡ biologic agent is living and replicating within a host
Contamination ≡ agent living on exterior surface of host
Silent Infection ≡ infection without disease (“commensal”)
Infectious disease ≡ infection accompanied by pathology
Pathos
Apr-19

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

5. 1. Agents (from large to small)
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)
Apr-19

6. 2. Reservoirs Reservoir ≡ where the agent multiplies and perpetuates
Types of reservoirs
Cases [symptomatic]
Carriers
Animals & insects
Inanimate objects
Apr-19

7. Carrier Reservoirs Carrier ≡ contagious without discernable signs
Types of carriers
Inapparent [throughout]
Incubatory
Convalescent
Apr-19

8. Animals Reservoirs (Zoonoses)
Zoonosis = an animal disease transmissible to humans
Types of zoonoses
Direct zoonosis: vertebrate animal  human (e.g., rabies)
Cyclozoonoses:  species #1 species #2  human  species #1 (e.g., Echinococcus tapeworm, right)
Apr-19
Source:

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

10. Inanimate Reservoirs
Water
Food
Soil
Other
Apr-19

11. Portals of Entry & Exit Respiratory Conjunctiva Urogenital
Gastrointestinal
Skin
Placenta
Etc.
Apr-19

12. Transmission by Contact
Direct (host  host)
Indirect (host  secretion  host)
Droplet (airborne, short distance)
Nuclei (airborne, suspended)
Apr-19

13. Transmission via Intermediaries
Vector (living)
Vehicles (inanimate)
Vectors types: mechanical, developmental, propagative, cyclopropagative.
Apr-19

14. Transmission Dynamics
Common source
Serial transmission
Apr-19

15. Transmission: Cycle in Nature
Each agent has its unique cycle in nature
Example: the blood worm (Shistosoma sp.).
Apr-19

16. Immunity
Immunity ≡ all factors that alter likelihood and severity of infection after host is exposed
Types of immunity (figure)
Apr-19

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

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

19. Interaction of Innate & Acquired Immunity
Apr-19

20. 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)
Apr-19

21. 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)
Apr-19

22. Herd Immunity Non-susceptible individuals represented by dark circles.
You do not need to vaccinate the entire herd to achieve infection control if the agent meets a lot of dead ends
Apr-19

23. Herd Immunity Animation
Apr-19