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, 1854The 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.

http://www.sfu.ca/gis/bguide/icons/figure1.4_snowmap.gif

Terms-1

• Acute, subacute, chronic– 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

4

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

• http://www.mansfield.ohio-state.edu/~sabedon/biol2040.htm– Excellent web site with definition of many, many

terms related to infection and disease.

5

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.

8The 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

http://www.humboldtredwoods.org/images/mosquito.jpg

10

Transmission• Microbe needs to get from reservoir to you.

• Contact– 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

http://students.washington.edu/grant/random/sneeze.jpg

WASH YOUR HANDS!!!!

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.

http://www.kennethkeith.com/milkgreeceb.JPG

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.

http://www.doktordoom.com/images/Tick.jpg

13

Koch’s Postulates

• Historical context:– So many germs, so many diseases– Logical framework for matching them

1.Microbe must be found in every instance of the disease.

2.Obtain microbe in pure culture

3.Produce disease in susceptible host

4.Re-isolate original microbe

http://www.nsc.gov.tw/files/popsc/2003_276/9208-13-04.jpg

14

Stages of Disease

• Incubation

• Prodromal• Illness

(acute/invasive)

• Decline

• Convalescent

http://www.goshen.edu/bio/NaSc104/graphics/stages.jpg

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

http://www.phac-aspc.gc.ca/publicat/ccdr-rmtc/03vol29/images/2906fg3.gif

http://whyfiles.org/016skeeter/images/malaria_distribution_2000.gif

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

http://www.cdc.gov/mmwr/preview/mmwrhtml/figures/00000122.gif

17

Herd ImmunityHerd 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.

http://www-micro.msb.le.ac.uk/3035/3035pics/Herd.gif

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