Disease Frequency Measures

functions of epidemiology

• identify the causes of disease 

• determine the extent to which disease occurs in populations

• understand the natural history of disease

• evaluate preventive and therapeutic interventions

• provide a foundation for developing health policy

etiology

cause(s) of disease

prognosis

understand the natural history of disease

john snow

• father of modern epidemiology

• investigated outbreak of cholera in england

communicable diseases

illnesses that spread from one person to another thru various modes of transmission

non-communicable diseases

lifestyle related diseases (ex: diabetes, obesity, HTN, CVD)

ignaz philip semmelweis

• pioneer of antiseptic procedures

• attributed to differences in maternal mortality to hand hygiene

non-recurrent disease

permanent health condition (ex: alzheimers disease)

recurrent disease

can have disease/illness several times over their life (ex: UTI)

immunity model

person gets illness then they become immune (ex: chicken pox)

susceptibility

possibility to develop the disease/experience a disease event

primary prevention

preventing the initial development of a disease (ex: immunizations)

secondary prevention

early detection of existing disease to reduce severity (potential to return to health) - ex: screening for cancer

tertiary prevention

reducing the impact of the disease (ex: rehab for stroke)

population at risk can be defined based on…

• geographic scope 

• personal attributes

• time period

direct transmission

based on physical contact

indirect (intermediate) transmission types

• vector (vector-borne transmission)

• vehicle

  • airborne

vector (vector-borne transmission): indirect transmission

• agent (microorganism, person, or animal) that carriers/transmits a pathogen

• often insect (ex: mosquitos/malaria, deer ticks, and lyme disease)

vehicle transmission: indirect transmission

inanimate object that transmits a pathogen (ex: food-borne outbreaks)

airborne transmission: indirect transmission

transmission via aerosols (ex: flu)

horizontal transmission

disease transmission person to person by either direct or indirect transmission

vertical transmission

disease transmission from parent to offspring (ex: HIV mom to baby during birth)

incubation period

• time period between an individual is exposed to a pathogen and the onset of symptoms/signs of disease

• ranges based on disease (ex: flu is days v mono is months)

herd immunity

widespread immunity of the population reduces the transmission of the disease among the few who are not immune above and beyond what might be expected 

endemic

habitual presence (expected amount) of disease within a geographic area - in other words the usual occurrence of a disease in a given area

epidemic

occurrence of illness in a population clearly in excess of normal expectancy

pandemic

worldwide epidemic

outbreak

less formal word but it means when there is more cases than normal but not yet enough to classify as a formal epidemic

proportion

a part in relation to a whole

rate

a quantity related to some unit of time

proportion formula

# of people with disease / total population

rate formula

measure in units/measure of time

prevalence

• frequency of disease at a given point in time

• similar to proportion

incidence

• frequency of disease that develops over a period of time

• similar to rate

cumulative incidence formula

total people with disease/population

incide rate (incidence density)

total people with disease/person-years

food specific attack rate

# of people who got sick who ate the certain food / # of people who ate the food

mortality

• incidence of death due to a specific disease

• denominator is usually estimated based on a midyear population

• measure of burden (in the form of death) to society due to a specific disease

mortality formula

total disease specific deaths/general population

mortality rate

• incidence of deaths due to specific disease among general population per year

• conveys info regarding how lethal the disease is in the general population

• for lethal diseases mortality rate can serve as a surrogate for incidence

mortality rate formula

total deaths/general population

case fatality

• incidence of deaths among those who have the specific disease

• conveys prognostic information regarding how lethal the disease for those who have it

case fatality formula

total deaths/population who have the disease

5 year survival rate

• useful for lethal diseases like cancer

• defined as the num of people with disease who are still alive 5 years after diagnosis

• typically expressed as a % (% of cases still alive at 5 years)

• more optimistic than survival rate

• sensitive to changes in early detection (diagnosis)

• doesn’t tell the complete story of what happened throughout the 5 years