1. Epidemiology and Disease terms
Epidemiology is the study of disease:
the study of the factors involved in the frequency and spread of disease.
Etiology
The study of the causes of disease
Signs and symptoms
Signs are observable/measurable
Symptoms are experienced by patient
Syndrome: collection of signs and symptoms

2. More epidemiology definitions
Incidence: # of new cases during a time period
Prevalence: ongoing cases at any one time
Both often expressed per number of people at risk
Mortality and morbidity rate:
Death and sickness per total population over a given period of time.
Many diseases have unique patterns, such as flu occurring during the winter months.

3. The first epidemiological study
John Snow, London, 1854
The cause of cholera wasn’t known, but Snow guessed it was spread by water. By mapping all the known cases and comparing them to the sources of water, he correctly identified the source of the problem.

4. Terms-1 Acute, subacute, chronic Local, systemic
Refers to how long the disease lasts
Acute is short lived, subacute longer,chronic longest
Local, systemic
A local infection is in one specific place, systemic means throughout the body.
Primary, secondary, superinfection
Primary: the first or main infection
Secondary: a second infection once weakened by a first one.
Second infection usually blamed on antibiotic Rx

5. Terms-2 Bacteremia, viremia vs. septicemia (blood poisoning)
Bacteremia is a clinical finding: bacteria in the blood
Septicemia is a disease condition, microbes actively growing in, infecting blood.
Viremia: viruses in the blood, common in systemic infections
Excellent web site with definition of many, many terms related to infection and disease.

6. Susceptibility and transmission
Exogenous vs. endogenous
Exogenous means infection comes from outside the host. Examples: common cold, STD.
Endogenous means host is original source of infection. Examples: boils on skin spread by fingers from Staph sinus infection; E. coli urinary tract infection.
Compromised host
Weakened immune system, poor lung clearance from bed rest, surgical wounds, bedsores, etc.

7. Where germs live: reservoirs
Humans
May be sick, may be carriers
Animals
Many diseases are zoonotic but can also be caught by humans; animal may be healthy or not.
Non-living
Soil, water are typical homes to microbes that can cause disease.

8. The Ins and Outs of infections: Portals of entry and exit
The successful parasite has to get in, multiply, then get out to spread to others.
People have lots of entry points
Glands, follicles that open to outside
Larger openings: respiratory system, digestive system, genito-urinary tract
Cuts, bites, burns, surgical incisions
Crossing the placenta, infecting the fetus
Pathogens have favored portals of entry
Some microbes aren’t harmful if portal is not optimal

9. Exit
The portal of exit is often the same or connected to a portal of entry
Coughing sneezing speaking: from mouth
Excretory systems: GI tract, in feces; in urine
From sex: vaginal fluid, semen
From blood: insect bites, shared needles

10. Transmission Contact Microbe needs to get from reservoir to you.
Direct contact: touching, kissing, sex, endogenous spread (one part of you to another)
“vertical”: mother to offspring, e.g. transovarian
Indirect contact, via fomites (inanimate objects)
Droplet transmission:
less than 1 meter thru air
WASH YOUR HANDS!!!!
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11. Transmission-2 Vehicles
Water: various viruses, bacteria, protozoa, mostly that cause diarrhea and enter water supply.
Food: unpasteurized or contaminated food, either improperly grown, processed, or prepared.
Airborne: microbes attached to dust, skin flakes, dried mucus become aerosols, travel thru air.

12. Transmission-3 Vectors Typically arthropods (insects, ticks)
Mechanical vectors: simply spread disease, e.g. houseflies walking on feces, spread germs to humans.
Biological: pathogen goes through part of life cycle in vector
Viruses or protozoa that reproduce within mosquito, e.g. Major method for spread of zoonoses.

13. Koch’s Postulates Historical context:
So many germs, so many diseases
Logical framework for matching them
Microbe must be found in every instance of the disease.
Obtain microbe in pure culture
Produce disease in susceptible host
Re-isolate original microbe

14. Stages of Disease Incubation Prodromal Illness (acute/invasive)
Decline
Convalescent
Stages for acute disease; chronic lasts longer.

15. Distribution of diseases in time and space
Some diseases, such as influenza, are seasonal.
Some diseases are found only in some geographic areas

16. Frequency of disease terms
Endemic: continually present in low numbers
Epidemic: higher than normal occurrence
Pandemic: epidemic spreads worldwide
Sporadic: cases show up only occasionally

17. Herd Immunity
Herd immunity: indirect protection from disease due to there being a high number of immune individuals in the population. The more immune individuals, the harder it is for the disease to be spread among many people; the cycle of transmission is broken.

18. Notifiable diseases
Notifiable disease: potentially harmful diseases which must be reported to the CDC or other Health Unit by physicians
Allows CDC to keep track and intervene.
Some examples listed by Arkansas Dept. of Health:
Anthrax, Botulism, Hepatitis A,
Pertussis, Plague, Q Fever, SARS, Smallpox,
Tularemia, Typhus,
Viral Hemorrhagic Fevers