:the presence and replication of microorganisms in the tissues of a host, accompanied by an immune response
Subclinical Infection
: a reaction by the host to the microorganism that is limited to an immune response that can only be detected through serologic or immunologic testing
Disease
: the clinical expression of infection, i.e. symptoms and signs
Colonization
: the presence of a replicating micoorganism without clinical or subclinical infection or disease
Contamination
: the presence of microorganisms on a body surface or tissue without invasion or reaction
Carrier
: a person colonized with an pathogen who shows no evidence of disease; disease may have been present earlier
Transmission
: the spread of infection from one person to another
Modes of Transmission of Infectious Diseases
Direct Contact
: transmission via proximity (not necessarily touching; e.g., STDs, hepatitis A)
Indirect Contact
: transfer of microorganism indirectly, usually through inanimate object
Common Vehicle
: a single inanimate vehicle serves to transmit the infectious agent to multiple hosts; food or water most common, but also blood supply, IV fluids, and medications
Airborne
: bugs carried in water droplets or dust; considered direct contact if within one meter of the host
Vector-borne
: bugs are carried by creepy-crawly things (flies, fleas, mosquitoes, ticks, snails, lions, tigers, bears)
Definitions for Infectious Disease Epidemiology
Epidemic
: sudden increase in the incidence or prevalence of an infection above previous or expected levels
Sporadic Disease
: occasional cases of disease at irregular intervals
Endemic Disease
: low-level frequency of infection or disease at regular intervals
Pandemic
: epidemic disease or infection that has spread among continents
Types of Epidemics
Common Source Outbreaks
: originates from a single source with which all affected cases have contact. (e.g., contaminated water supply and cholera, HIV in national blood supply)
Point Epidemics
: single source; all cases have contact during a defined interval of time. (e.g., food poisoning)
Progressive epidemics
: pathogen is spread host to host (e.g., HIV, TB, Ebola); source is often undefined
Investigating Epidemics
Several questions to be answered:
is there an epidemic?
what organism is causing the outbreak?
where is the source?
the source is often the key to controlling the epidemic
Steps to investigating an epidemic
(1) Define problem: characterize the illness (clinical picture, time/location of reported cases, assess similarities)
at this point you often have to come up with some initial recommendations for control because the public cannot wait until you’re done with your study
ex: someone notices more than the average number of hepatitis cases at the VA
(2) Develop case definition: this is a description of the illness that allows standardization between investigators
may change over time: i.e., AIDS Þ initially defined by pneumocystis infection, Kaposi’s sarcoma; later defined by other AIDS-defining conditions; then by HIV virus; then by CD4+ count < 200, etc
ex: define hepatitis as Hep B, based on serologic studies
(3) Appraise existing data: try to find similarities between cases (time of onset, place, demographics, ID the bug)
ex: you find that all cases occurred on ward C, and that most affected patients are diabetic
(4) Formulate hypothesis regarding cause of epidemic: try to elucidate the source, mode of spread, etc.
ex: could be needle spread, contaminated insulin, spring-loaded needle
(5) Develop study to test each hypothesis: cross-sectional, case-control study, cohort, etc
(6) Analyze the data, review results, and draw conclusions
ex: data shows odds ratio of 7.2 for patients that use spring-loaded needles Þ don’t share needles!
Controlling Epidemics
Three ways to do it:
(1) control the source: if you can find it, it’s the easiest way to do it
(2) interrupt transmission: unidentified source as in propagated epidemic
Þ control spread; easiest in direct spread
(3) modify the host response: vaccines
The Message: identifying and controlling epidemics requires a good public health infrastructure with channels for reporting cases and possible outbreaks, and a means of controlling them (give more money to the CDC)