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The amount of a particular disease that is usually present in a community is referred to as the baseline or endemic level of the disease. This level is not necessarily the desired level, which may in fact be zero, but rather is the observed level. In the absence of intervention and assuming that the level is not high enough to deplete the pool of susceptible persons, the disease may continue to occur at this level indefinitely. Thus, the baseline level is often regarded as the expected level of the disease.
While some diseases are so rare in a given population that a single case warrants an epidemiologic investigation (e.g., rabies, plague, polio), other diseases occur more commonly so that only deviations from the norm warrant investigation. Sporadic refers to a disease that occurs infrequently and irregularly. Endemic refers to the constant presence and/or usual prevalence of a disease or infectious agent in a population within a geographic area. Hyperendemic refers to persistent, high levels of disease occurrence.
Occasionally, the amount of disease in a community rises above the expected level. Epidemic refers to an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area. Outbreak carries the same definition of the epidemic but is often used for a more limited geographic area. A cluster refers to an aggregation of cases grouped in place and time that are suspected to be greater than the number expected, even though the expected number may not be known. Pandemic refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people.
There are several changes that may occur in an infectious agent that may trigger an epidemic. These include::55
- Increased virulence
- Introduction into a novel setting
- Changes in host susceptibility to the infectious agent
An epidemic disease is not required to be contagious, and the term has been applied to West Nile fever and the obesity epidemic (e.g. by the World Health Organisation), among others.
The conditions which govern the outbreak of epidemics include infected food supplies such as contaminated drinking water and the migration of populations of certain animals, such as rats or mosquitoes, which can act as disease vectors.
Certain epidemics occur at certain seasons. For example, whooping-cough occurs in spring, whereas measles produces two epidemics, one in winter and one in March. Influenza, the common cold, and other infections of the upper respiratory tract, such as sore throat, occur predominantly in the winter. There is another variation, both as regards the number of people affected and the number who die in successive epidemics: the severity of successive epidemics rises and falls over periods of five or ten years.
Common source outbreak
In a common source outbreak epidemic, the affected individuals had an exposure to a common agent. If the exposure is singular and all of the affected individuals develop the disease over a single exposure and incubation course, it can be termed a point source outbreak. If the exposure was continuous or variable, it can be termed a continuous outbreak or intermittent outbreak, respectively.:
In a propagated outbreak, the disease spreads person-to-person. Affected individuals may become independent reservoirs leading to further exposures.:56
Many epidemics will have characteristics of both common source and propagated outbreaks (sometimes referred as mixed outbreak).
- Airborne transmission: Airborne transmission is the spread of infection by droplet nuclei or dust in the air. Without the intervention of winds or drafts the distance over which airborne infection takes place is short, say 10 to 20 feet.
- Arthropod transmission: Arthropod transmission takes place by an insect, either mechanically through a contaminated proboscis or feet, or biologically when there is growth or replication of an organism in the arthropod.
- Biological transmission: Involving a biological process, e.g. passing a stage of development of the infecting agent in an intermediate host. Opposite to mechanical transmission.
- Contact transmission: The disease agent is transferred directly by biting, sucking, chewing or indirectly by inhalation of droplets, drinking of contaminated water, traveling in contaminated vehicles.
- Cyclopropagative transmission: The agent undergoes both development and multiplication in the transmitting vehicle.
- Developmental transmission: The agent undergoes some development in the transmission vehicle.
- Fecal-oral transmission: The infectious agent is shed by the infected host in feces and acquired by the susceptible host through ingestion of contaminated material.
- Horizontal transmission: Lateral spread to others in the same group and at the same time; spread to contemporaries.
- Propagative transmission: The agent multiplies in the transmission vehicle.
- Vertical transmission: From one generation to the next, perhaps transovarially or by intrauterine infection of the fetus. Some retroviruses are transmitted in the germ line, i.e. their genetic material is integrated into the DNA of either the ovum or sperm.
Epidemics occur when an agent and susceptible hosts are present in adequate numbers, and the agent can be effectively conveyed from a source to the susceptible hosts. More specifically, an epidemic may result from:
- A recent increase in amount or virulence of the agent,
- The recent introduction of the agent into a setting where it has not been before,
- An enhanced mode of transmission so that more susceptible persons are exposed,
- A change in the susceptibility of the host response to the agent, and/or
- Factors that increase host exposure or involve introduction through new portals of entry.(47)
- Crimean-Congo haemorrhagic fever
- Ebola virus disease
- Hendra virus infection
- Influenza (pandemic, seasonal, zoonotic)
- Lassa fever
- Marburg virus disease
- Nipah virus infection
- Novel coronavirus (2019-nCoV)
- Rift Valley fever
- Yellow fever
- Zika virus disease
- Haemophilus infection
- Haemophilus influenzae type b (HiB) infection
- Haemorrhagic fever
- Haemorrhagic fever with renal syndrome (Puumala caused)
- Hantavirus infection
- Health communication
- Health inequality
- Healthcare-associated infections
- Hepatitis A
- Hepatitis B
- Hepatitis C
- Hepatitis E
- HIV infection
- Human immunodeficiency virus infection
- Human papillomavirus infection
- Infections in acute care hospitals
- Infections in intensive care units
- Infections in long-term care facilities
- Infectious diseases on aircrafts
- Influenza A (H9N2)
- Influenza in humans, avian origin
- Influenza in humans, pandemic
- Influenza in humans, seasonal
- Influenza in humans, swine origin
- Invasive Haemophilus influenzae disease
- Invasive meningococcal disease
- Invasive pneumococcal disease
- S. pneumoniae
- Sandfly-borne diseases
- Schmallenberg virus
- Scientific advice
- Seasonal influenza
- Severe acute respiratory syndrome (SARS)
- Sexually transmitted infections
- Sindbis fever
- Streptococcus pneumoniae
- Surgical site infections
- Swine-origin influenza
- Syphilis, congenital