Session 4 - infectious disease epidemiology

Question 1

Infectious disease definition

Answer 1

Disease caused by transmissible agents that replicate in the affected host

Question 2

Infection occurs when

Answer 2

A susceptible host is exposed to, and acquires, the agent

Question 3

Infection can be acquired from

Answer 3

environmental sites or from other hosts

Question 4

Direct transmission - types of contact

Answer 4

Sexual, transplacental, droplets

Question 5

Indirect transmission - types of transmission

Answer 5

Environmental - Water borne, Food borne, Air borne
Vector - Biological (mosquitos- malaria), Mechanical (surfaces, flies)

Question 6

Pathogen - Periods

Answer 6

Latent Period - establishment, replication within-host, cannot transmit
Infectious Period - transmission possible, shedding and onward transmission

Question 7

Host - Periods

Answer 7

Incubation Period - the time before becoming symptomatic and (possibly) help-
seeking behaviour
Symptomatic Period - Sick, seek care

Question 8

Generation time (serial interval)

Answer 8

Time between linked infections - will be a range
From first person showing symptoms, to next person showing symptoms
Can be negative if long and variable latency period

Question 9

Attack rate

Answer 9

How many at risk people become infected / ill?
Cumulative incidence
%
eg 37% of people at risk of developing the disease
developed the disease

Question 10

Secondary attack rate

Answer 10

Proportion of contacts with a primary case that become infected
%
eg On average, 23% of people in contact with an infected
person became infected themselves

Question 11

Reproductive number (R0) - Basic Reproductive number

Answer 11

Average number of new infections caused by a single host in a completely naive population

Question 12

R(e) Effective reproductive number

Answer 12

Average number of secondary infections produced when one infected individual is introduced into a ‘real’ population including some immune

Question 13

Endemic infection

Answer 13

Pathogen and host at equilibrium - can be high level, low, or seasonal incidence

Question 14

Epidemic infection

Answer 14

Rate of infection / number of cases in excess of the expected number

Question 15

Point source epidemic

Answer 15

All cases arise from single source
Often indirect transmission through the environment
Can reflect distribution of incubation period
eg food poisoning, water contamination ,legionella

Question 16

Propagated epidemic

Answer 16

Transmission between cases and susceptibles
eg Measles, HIV
Gradually increasing peaks, may widen and coalesce

Question 17

Force of infection

Answer 17

Rate at which susceptibles acquire infection
Can be estimated from case notification or serological data

Question 18

Who are the hosts of an infectious disease?

Answer 18

Individuals eg people, animals, plants
Cells within an individual, target cells, immune cells
Clusters of individuals - lakes, herds, pens, households

Question 19

Zoonotic infections

Answer 19

Spill over into humans from a Reservoir
Contact, vector or Environmental transmission
eg Rabies, lyme disease, plague, ebola

Question 20

Indirect transmission by a vector

Answer 20

Mediated by another organism
Mechanical carrier
or - biological - infection of vector eg mosquitos - plasmodium parasite complete life cycle in mosquito

Question 21

Length of the latency period relative to the length of the incubation period?

Answer 21

When the latency period is shorter than the incubation period, there is a greater
potential for spread as infected cases are able to transmit the disease but do not
display any signs of being ill. In the absence of a screening test, isolation and/or
treatment of affected people is more difficult to implement. Infected people are not
disclosed by their symptoms early enough to allow intervention to disrupt the
transmission cycle