Download A Glossary of Epidemiology Terms

A Glossary of Epidemiology Terms
1. Aetiology (Etiology)
(The study of) the causes of disease.
2. Acquired immunity
The inherited potential to resist a disease or infection. See also immunity.
3. Analytic Epidemiology
A focused study of the determinants of disease or reasons for high or low frequency of disease
in specific groups.
4. Antibody
A protein produced in the blood of vertebrates following exposure to an antigen. The antibody
binds specifically to the antigen and thus stimulates its inactivation by other parts of the immune
system.
The major classes of antibody are immunoglobulin A, or IgA, found predominantly in bodily
secretions like saliva; IgM and IgG which are typically produced sequentially in response to
microparasitic infections; and IgE which is often elevated in the response to helminth infections.
Only IgG is able to cross the placenta to provide maternal immunity.
5. Antigen
A protein, typically foreign, that elicits a specific immune response.
6. Anthelmintic
A drug, for example praziquantal, used specifically against helminth or worm infections.
7. Arbovirus
A virus which uses Arthropods as vectors and is transmitted in their saliva to the definitive host.
For example, yellow fever.
(From Arthropod borne virus.)
B
8. B cell
A white blood cell which produces antibody.
9. Bacteraemia
The presence of bacteria in the blood.
C
10. Carrier
An individual who is infected but has no symptoms of disease.
There are two types of carrier state: silent carriers retain their infectiousness, while latent
carriers are not infectious. For example, many of those infected with tuberculosis are silent
carriers, while infection with herpes virus may create latent carriers.
11. Case-control study
A study in which the risk factors of people with a disease are compared with those without a
disease.
12. Chemotherapy
Drug treatment of a diseased individual.
13. Chemoprophylaxis
Drug treatment designed to prevent future occurrences of disease. Treatment may be
chemotherapy as far as an individual is concerned but chemopropylactic for the population as a
whole.
14. Classical epidemiology
Our term for the varieties of epidemiology primarily concerned with the statistical relationships
between disease agents, both infectious and non-infectious; for example a study to establish the
relative risk of lung cancer associated with smoking.
We contrast this with ecological epidemiology.
15. Close contact infection
An infection which requires close contact, other than sexual contact, between susceptible and
infectious individuals, for transmission.
16. Cohort
A subsection of a population with a common feature, usually age. For example, all those
individuals in the UK born in 1964 form a birth cohort.
17. Contact rate
The rate at which susceptible meet infecteds. Usually measured as individuals per unit time.
18. Cross-sectional study
Same as a horizontal study.
19. Crude birth rate
The number of live births in a year divided by the population size.
20. Crude death rate
The number of deaths in a year divided by the population size.
D
21. Definitive host
The host in which a parasite reproduces sexually.
22. Direct life cycle
A life cycle in which a parasite is transmitted directly from one host to the next without an
intermediate host or vector of another species.
E
23. Ecological epidemiology
A branch of epidemiology which views disease as a result of the ecological interactions between
populations of hosts and parasites. We contrast this with classical epidemiology.
24. ELISA
A technique using the antigen binding properties of antibodies to detect specific antigens or
antibodies. Visualization is typically made possible by enzyme induced color formation.
From Enzyme Linked ImmunoSorbent Assay.
25. Endemic
A term to describe levels of infection which do not exhibit wide fluctuations through time in a
defined place.
For microparasites like measles, the term is used slightly differently to indicate an infection
which can persist in a population in the long term without needing to be reintroduced from
outside.
26. Epidemic
A rapid increase in the levels of an infection. Typical of the microparasitic infections (with long
lasting immunity and short generation times) an epidemic is usually heralded by an exponential
rise in the number of cases in time and a subsequent decline as susceptible numbers are
exhausted. Epidemics may arise from the introduction of a novel pathogen (or strain) to a
previously unexposed (naive) population or as a result of the regrowth of susceptible numbers
some time after a previous epidemic due to the same infectious agent. Contrast endemic,
pandemic.
27. Epidemiology
The study of the distribution and determinants of diseases and injuries in human (or animal)
host populations.
28. Epizootic
An epidemic in an animal host population. (But while there may be scientific reasons for
distinguishing a human epidemic from an epizootic, the use of epizootic is not required on
linguistic grounds any more than a demographer is required to count only people:
29. Epiphytotic
An epidemic in a plant host population.
30. Helminths
Worms, in particular the five groups of parasitic worms: Monogeneans (flukes), Digeneans
(flukes, schistosomes), Cestodes (tapeworms), Nematodes (roundworms) and
Acanthocephalans (spiny-headed worms).
31. Herd immunity
Specifically, the mechanism by which an infection may be eradicated from a population although
some susceptibles still remain, because the remainder of the population is immune and thus
transmission is reduced.
More generally, the immunological status of a population of hosts and its effect on transmission
rates.
32. HI test
Hemagglutination inhibition test, a serological test used to detect antibodies specific to a
particular family of viruses which possess the ability to agglutinate red blood cells e.g. measles,
rubella and influenza.
33. Horizontal transmission
Transmission occurring generally within a population, but not including vertical transmission.
I
34. Immunity
1) A state in which a host is not susceptible to infection or disease, or
2) the mechanisms by which this is achieved. Immunity is achieved by an individual through one
of three routes: natural or innate immunity genetically inherited or acquired through maternal
antibody, acquired immunity conferred after contact with a disease, and artificial immunity after
a successful vaccination. Also termed specific immunity, resistance or specific resistance.
Specific immunity is divided into cellular immunity acting via the direct involvement of T cells
and humoral immunity involving antibodies and B cells.
35. Immunogenicity
The ability of a vaccine to stimulate the immune system, as measured by the proportion of
individuals who produce specific antibody or T cells, or the amount of antibody produced, say.
Not the same as efficacy.
36. Immunopathology
Damage to the host caused by its own immune response against a pathogen.
37. Immunosuppression
A reduction in the capacity of the immune system. Caused by infection (e.g. HIV), drug
treatment, pregnancy and malnutrition among others. Immunosuppressed individuals are
commonly referred to as immunocompromised.
38. Incidence
The rate at which new cases of infection arise in a population. Classically measured as an
attack rate.
39. Incubation period
The time that elapses between infection and the appearance of symptoms of a disease. Not the
same as the latent period.
40. Indirect life cycle
A life cycle which requires one or more intermediate hosts before the definitive host species is
reinfected. Compare direct, nondirect.
41. Indirect transmission
Transmission of a parasite through an indirect life cycle.
42. Infection
The presence of a parasite within a host where it may or may not cause disease.
43. Infectious period
The time period during which infecteds are able to transmit an infection to any susceptible host
or vector they contact. Note that the infectious period may not necessarily be associated with
symptoms of the disease.
44. Intermediate host
See vector.
L
45. Latent period
The time from infection to when the individual is infectious to others. In helminths it is termed the
pre-patent period. Not the same as the incubation period.
46. Longitudinal study
A study taking place over time. If individuals are followed, this is a longitudinal cohort study. If
individuals are not followed, but classes (usually age classes) are restudied, this is a
longitudinal cross-sectional study.
M
47. Macroparasites
Typically, the parasitic helminths and arthropods. In general, parasites which do not multiply
within their definitive hosts but instead produce transmission stages (eggs and larvae) which
pass into the external environment. Immune responses elicited against macroparasites
generally depend on the number of parasites present in a given host and tends to be of a
relatively transient nature. The key epidemiological measurement is generally the number of
parasites per host.
Such parasites are often found in a highly aggregated distribution.
Contrast microparasites.
48. Maternal immunity
Immunity for a neonate provided by IgG antibody generated by a mother and passed across the
placenta to the unborn offspring. This provides short lived protection (with a typical half life of 36 months) to the neonate. Also known as immunity.
49. Mathematical model
A formal framework to convey ideas about the components of a host-parasite interaction.
Construction requires three major types of information: (a) a clear understanding of the
interaction within the individual host between the infectious agent and the host, (b) the mode
and rate of transmission between individuals, and (c) host population characteristics such as
demography and behavior.
Mathematical models can aid exploration of the behavior of the system under various conditions
from which to determine the dominant factors generating observed patterns and phenomena.
They also aid data collection and interpretation and parameter estimation, and provide tools for
identifying possible approaches to control and for assessing the potential impact of different
intervention measures.
50. Microparasites
Typically, viruses, bacteria, fungi and protozoa. More generally, parasites which multiply within
their definitive hosts. Microparasites are characterized by small size, short generation times,
and a tendency to induce immunity to reinfection in those hosts that survive. The duration of
infection is usually short in relation to the lifespan of the host, but there are important
exceptions, such as the slow viruses. The key epidemiological variable, by contrast with
macroparasites, is whether or not the individual host is infected.
51. Morbidity
State of ill-health produced by a disease.
52. Mortality rate
The per capita death rate in a population. The mortality rate is the reciprocal of the population
life expectancy.
53. Multiple infection
An infection in which an individual is infected by parasites of more than one species.
N
54. Notifiable disease
Diseases, usually of an infectious nature, whose occurrence is required by law to be made
known to a health officer or local government authority.
55. Non-direct transmission
A mode of transmission that differs in some mysterious way from indirect transmission; we don't
make a distinction in this course.
O
56. Odds ratio
A comparison of the presence of a risk factor for disease in a sample of diseased subjects and
non diseased controls. The number of people with disease who were exposed to a risk factor
(I.e.) over those with disease who were not exposed (Io) divided by those without disease who
were exposed (Ne) over those without who were not exposed (No). Thus OR=(I.e./Io)/(Ne/No)=
I.e. No / Io Ne. This measure should be used for case control studies where we retrospectively
look at risks in those with and without disease. Also known as exposure odds ratio.
P
57. Pandemic
An epidemic widely distributed in space.
58. Panzootic
A widely distributed epizootic, often one affecting more than one host species.
59. Passive immunity
Immunity which has been acquired through the transfer of maternal or other antibodies. Passive
immunization does not induce immunological memory.
60. Parasite
1) Any disease causing organism. 2) An organism exhibiting an obligatory dependence on
another organism, its host, which is detrimental to the host.
61. Pathogen
Same as a parasite.
62. Pathogenicity
The degree to which a pathogen debilitates its host.
63. Pre-patent period
The time from infection until when a female starts to produce eggs in helminth infections,
equivalent to latent period in micro-parasitic infections.
64. Predator
An animal that kills its prey and then feeds on it to subsist until the next kill.
65. Prevalence
The proportion of the host population infected (or with some marker of past or present infection)
at a particular time.
66. Prospective study
A study in which people are initially enrolled and then followed up at subsequent times.
67. Protozoan
A unicellular eukaryote.
R
68. Rate
The number of events happening divided by the length of time over which they happen. A rate
of change is the amount of change happening in a interval divided by the length of the interval;
for small intervals the rate of change might be given by a simple rule (a differential equation).
69. Reproductive ratio
(1) Basic reproductive ratio, Ro, a dimensionless parameter which encapsulates the biological
details of different transmission mechanisms. For microparasites, Ro, is defined as the average
number of secondary cases of infection to which one primary case gives rise throughout its
infectious period if introduced into a defined population consisting solely of susceptible
individuals. For macroparasites, Ro, is the average number of female offspring (or just offspring
in the case of hermaphroditic species) produced throughout the lifetime of a mature female
parasite, which themselves achieve reproductive maturity in the absence of density-dependent
constraints on the parasite establishment, survival or reproduction.
Also known as the basic reproduction rate, number.
(2) Effective reproductive ratio, R, The number of secondary cases (microparasites) or female
offspring (macroparasites) produced in a host population not consisting entirely of susceptible
individuals (microparasites) or within which density dependent constraints limit parasite
population growth (macroparasites). Under conditions of stable endemic infection, R=1.
70. Relative risk (RR)
The proportion of diseased people amongst those exposed to the relevant risk factor divided by
the proportion of diseased people amongst those not exposed to the risk factor. This should be
used in those cohort studies where those with and without disease are followed to observe
which individuals become diseased.
71. Resistance
1) The reduction, due to genetic selection, of susceptibility of a parasite or its vector to
chemotherapy.
2) The ability of a host to resist a pathogen. Compare immunity.
72. Retrospective study
A study in which people are enrolled and then have their history of risks, infections or disease
measured.
S
73. Sensitivity
The ability of a test to work on people you know have the infection. More precisely TP/(TP+FN),
where TP is the number of true positives and FN is the number of false negatives
74. Serology
The study of antigen-antibody reactions. More generally, the use of serotype data to infer an
individual's history of infection.
75. Seropositive
n., an individual whose serotype suggests that they have experienced infection in the past.
76. Seroprevalence
The proportion of a population who are seropositive.
77. Serotype
The range of antibodies which an individual possesses, usually based on sampling from blood
serum or saliva.
Different strains of a pathogen can sometimes be distinguished by the different antibodies they
induce in a host, or with which they can be made to react in vitro; thus the word serotype has
also come to be applied to a particular strain (`the virulent serotype'). This is the more common
clinical usage.
The range of antibodies used to define a serotype obviously depends on those available to the
researcher. Sometimes, as for measles, the presence of a known antibody within the serum of
an individual correlates extremely well with the clinical observation that that individual is
protected against any further infections. But sometimes, as for malaria, there is as yet no
definite relationship between a given serotype and the presence of a functional immunity, which
may make the use of the word serotype unhelpful when trying to distinguish between different
parasites for the purposes of understanding transmission.
78. Specificity
The ability of a test to fail on people you know don't have the infection. More precisely
TN/(TN+FP), where TN is the number of true negatives and FP is the number of false positives
79. Subclinical infection
An infection in which symptoms are sufficiently mild or unapparent to escape diagnosis other
than by positive confirmation of the ability to transmit the infection or serologically.
80. Susceptible
An individual accessible to or liable to infection by a pathogen.
81. Symptom
A condition of the body reported by an individual when suffering from a disease; here used more
loosely to include signs: any evidence used in diagnosis or identification of infected individuals.
T
82. T cell
A white blood cell responsible for cell-mediated immunity in particular and immunoregulation in
general.
83. Transmission
The process by which a pathogen passes from a source of infection to a new host. There are
two major types: horizontal and vertical transmission. The majority of transmission processes
operate horizontally.
V
84. Vaccine
A drug intended to induce active artificial immunity against a pathogen. Vaccines may be live or
dead. Live vaccines are usually attenuated versions of the wildtype pathogen, such as the MMR
vaccines, which are strains of measles mumps and rubella viruses repeatedly passed through
cell lines until non-pathogenic. Typically, live vaccines need only be given as a single dose to
induce a full immunological response, inducing specific memory. Dead vaccines are either killed
whole parasite, as with the Salk polio strain and pertussis vaccine, or some highly immunogenic
fraction of the parasite, as in toxoid vaccines. Killed vaccines and toxoids which do not multiply
in the host must usually be administered in multiple doses to induce a full immunological
response.
Vaccination should be distinguished from passive immunization in which concentrated specific
antibodies which can be used therapeutically to abrogate an ongoing infection or to provide
short term protection (of the order of months), for example against hepatitis A. Passive
immunization does not induce immunological memory.
85. Vector
1) The intermediate hosts of parasites with indirect life cycles.
2) Anything which transmits parasites.
86. Vertical transmission
Vertical transmission occurs when a parent conveys an infection to its unborn offspring, as
occurs in syphilis in man or in many arboviruses of arthropods. Perinatal infection is a special
form of vertical transmission.
87. Viraemia
The presence of virus in the blood.
88. Virion
A mature and infectious virus particle.
89. Virulence
(1) The case mortality rate of an infection. (2) The extent to which a pathogen harms its host.
These are different usages: what they have in common is that they refer to the effect on an
already infected host, not to the degree of transmissibility to a subsequent susceptible.
Z
90. Zoonosis
A parasite naturally transmitted between man and other vertebrate species