6.1. Terms and Definitions
Communicable diseases (CD) (Syn: transmissible disease): A disease whose causal agent can be transmitted from successive hosts to healthy subjects, from one individual to another. An illness due to a specific infectious agent or its toxic products that arises through transmission of such agent or products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector, or the inanimate environment. All infections and infestations are communicable diseases; thus, not all communicable diseases are infectious diseases. A disease can be communicable (transmissible) but not contagious if it requires a vector for its transmission (e.g., tetanus). Thus, communicable diseases include contagious and noncontagious diseases. The latter include diseases genetically inherited, and diseases exclusively transmitted through vector (J. Last Dictionary).
Non-communicable diseases (NCD) It is a disease for which evidence is lacking that transmission from individual to individual is possible by contagion, a vector, biological heredity, or inheritance (J. Last Dictionary).
Syndromic surveillance for communicable diseases: A complex of signs and symptoms that tend to occur together, often characterizing a CD. The ongoing, systematic collection, analysis, interpretation, and application of real-time indicators for a group of signs and symptoms - syndromes, where fever is almost always present, suggesting a CD, that allow for detection before public health authorities might otherwise identify them etiologically.
Indicator-based surveillance (IBS) is defined as the systematic collection, monitoring, analysis, and interpretation of structured data, i.e., indicators, produced by a number of well-identified, predominantly health-based formal sources. The collection of IBS data is a routine, regular process which is primarily passive. Data are collected according to established case definitions which are either disease-specific or syndromic. They may be collected as individual or aggregated data and originate from either exhaustive or sentinel systems. Data are analyzed in comparison with baseline values and thresholds to determine unusual disease patterns. IBS sources of information are mainly health-based (e.g. health-care structures, health professionals, laboratories), but may also include structured non-human health sources such as animal health data such as zoonoses, environmental health and meteorological data, or entomological data when these are regularly collected and organized for human health purposes. IBS data are not only employed for EWAR purposes; they are used primarily for achieving other surveillance objectives such as measuring impact of programmes or the identification of priority health problems (WHO 2014).
Event-based surveillance (EBS) is defined as the organized collection, monitoring, assessment, and interpretation of mainly unstructured ad hoc information regarding health events or risks, which may represent an acute risk to human health. EBS is a functional component of EWAR. The information collected for EBS is diverse in nature and originates from multiple, often not predetermined , both official and unofficial, including rumors, stories, reports by the media or ad hoc reports from informal networks. The information collection process is mainly active and carried out through a systematic framework specifically established for EBS purposes (WHO 2014). The goal of EBS is to detect unusual events that might signal an outbreak or any acute public health problem, including health services. EBS can also be community-based, meaning that information is reported by people in the community through a hotline or other messaging system.
Acute Public Health Event (WHO) Any event that represents immediate threat to human health and requires prompt action, i.e., implementation of control and/or mitigation measures to protect the health of the public independently of the origin or cause. This term includes events that have not yet led to disease in humans but have the potential to cause disease through exposure of humans to infected or contaminated food, water, animals, manufactured products, environments, or as a result direct or indirect consequences of natural events, conflicts, or other disruptions of critical infrastructure.
Clusters are aggregations of relatively uncommon events or diseases (syndromes) in space and/or time, in amounts that are believed or perceived to be greater than could be expected by chance. Putative disease clusters are often perceived to exist on the basis of anecdotal evidence, and much effort may be expended by epidemiologists and biostatisticians in assessing whether a true cluster of disease exists (J. Last Dictionary). For surveillance, clusters are a signal of something that should be verified, confirmed, and assessed, and after discarded or investigated, to know if there is a common cause and is necessary to implement prevention and control measures. Clusters can be detected using IBS or EBS.
Outbreak is an epidemic limited to localized increase in the incidence of a disease, e.g., in a village, town, or closed institution (J. Last Dictionary).
Epidemic is defined by the occurrence in a community or region of cases of an illness, specific health-related behavior, or other health-related events clearly in excess of normal expectancy. The community or region and the period in which the cases occur must be specified precisely. The number of cases indicating the presence of an epidemic varies according to the agent, size, and type of population exposed; previous experience or lack of exposure to the disease; and time and place of occurrence. A single case of a communicable disease long absent from a population or first invasion by a disease not previously recognized in that area, requires immediate reporting and full field investigation; two cases of such a disease associated in time and place may be sufficient evidence to be considered an epidemic (J. Last Dictionary).