Disease Transmission

January 21st, 2010

Basic Terms• Pathogen

– Microorganism that causes disease• Commensal

– Microorganisms that live in the human or animal without affecting them

• Colonization– Living bacteria are present in the host– You can be colonized but not infected– Example: antibiotic resistant Staphylococcus aureus

• Infection– Invasion by microorganisms that starts the disease

process

Basic Terms• Incubation period

– Time between infection and appearance of symptoms

• Prodromal period– Time when symptoms are mild

• Morbidity– Illness

• Mortality– Death

Basic Terms• Fomite

– Inanimate object that transmits disease (your cell phone!)

• Vehicle– Medium that transmits disease– Water– Food– Air– Blood

Basic Terms• Vector

– Arthropods that carry infectious disease– The microorganism can multiply in the vector

• Reservoir– Any site where an infectious agent survives– Animal– Natural environment (water, soil)– Diseases can be acquired from their reservoirs– A reservoir does not have to play a role in disease

transmission, but it provides a source of pathogens

Basic Terms• Zoonotic disease

– Disease that can be transmitted from humans to animals

– Some very important environmentally transmitted diseases are zoonotic

– Transmission cycles of many diseases go on naturally among animals until we blunder into the cycle

Epidemiology terms• Incidence

Number of new cases of diseasePerson-time at risk

• Incidence always incorporates time- the duration of observation

Epidemiology terms• Prevalence

Number of new cases of diseaseNumber of people at risk

• Prevalence is at a single point in time

• A snapshot of the disease in the population

Epidemiology terms• Attack rate

Number of new infectionsPerson-time at risk of infection

• Attack rate is usually used during an epidemic

• Incidence during a limited period of time• Example: people who ate at a certain

restaurant

Epidemiology terms• Epidemic

– Large number of cases of a disease in a relatively short period

– An epidemic of measles in unvaccinated schoolchildren

• Endemic– Always present in a population– Malaria endemic regions of the world

Epidemiology concepts

• R0: The reproductive number• the average number of secondary

infections produced when one infected individual is introduced into a host population where everyone is susceptible

• infection can get started in a fully susceptible population if and only if R0 > 1

A model of disease transmission: The SIR model

• Common model for describing the transmission of infectious diseases

• Can be used for mathematical modeling of disease transmission, but is also a conceptual model

• Describes a population where individuals fall into three main categories:– Susceptible (S): can get disease– Infectious (I): Have disease, can spread to

others– Recovered (R): Can no longer spread disease

A model of disease transmission: The SIR model

• Different individuals within this population can be in one of a few key states at any given time– Susceptible to disease (S)– infectious/asymptomatic (I): transmission risk

because they do not know they are ill– infectious/symptomatic (I)– non-infectious/asymptomatic; recovered (R)

A model of disease transmission: The SIR model

• A dynamic model: individuals are moving from state to state over time

• This is key: the transmission of infectious diseases is a dynamic process– Within human populations– Within animal populations– Between humans, animals, and insect vectors– From humans to the environment and back to

humans

The SIR model: key detailsThere are two sets of variables:• Variables describing the states people are in

– S=susceptible– I=infectious– R=non-infectious/asymptomatic

• Variables describing how many people are moving between these states (parameters)– Example: γ=Fraction of people in state R

who move to state S

•S: Susceptible•I: Infectious (symptomatic+asymptomatic)•R: Non-infectious•W: Concentration of pathogens in the environment•β: Infection rate due to exposure to pathogen•δ: Fraction of people who move from state I to state R•γ: Fraction of people who move from state R to state S•Solid lines: Individuals moving from state to state•Dashed lines: Pathogen flows between individuals in different states

The SIR Model

ENVIRONMENTW

S R

I

Epidemiology concepts: Epidemic curves

• An epidemic curve can provide information on the following characteristics of an outbreak:– Pattern of spread– Magnitude– Outliers– Time trend– Exposure and/or disease incubation

period

Epidemic curves• The overall shape of the epi curve

can reveal the type of outbreak– Common source– Point source– Propagated

Epidemic curve: a point source outbreak

Example: Legionnaire’s Disease

Epidemic curve: a point source outbreak

A gastroenteritis outbreak in a nursing home

Epidemic curve: a common source outbreak

a foodborne outbreak from a widely distributed food product

Epidemic curve: a propagated outbreak

SARS in Toronto: A propagated outbreak with two waves of cases