A quotient (rate), with the number of new cases of a specified disease diagnosed or reported during a defined period of time as the numerator, and the number of persons in a stated population in which the cases occurred as the denominator. This is usually expressed as cases per 1,000 or 100,000 per annum. This rate may be expressed as age- or sex-specific or as specific for any other population characteristic or subdivison (see Morbidity rate and Prevalence rate). Attack rate, or case rate, is an incidence rate often used for particular groups, observed for limited periods and under special circumstances, as in an epidemic, usually expressed as percent (cases per 100). The secondary attack rate in communicable disease practice expresses the number of cases among familial or institutional contacts occurring within the accepted incubation period following exposure to a primary case, in relation to the total of determinable. Infection rate expresses the incidence of all infections, manifest and inapparent.
morbidity rate
An incidence rate used to include all persons in the population under consideration who become clinically ill during the period of time stated. The population may be limited to a specific sex, age group or those with certain other characteristics.
mortality rate
A rate calculated in the same way as an incidence rate, using as a numerator the number of deaths occurring in the population during the stated period of time, usually a year. A total or crude mortality rate utilizes deaths from all causes, usually expressed as deaths per 1,000, while a disease-specific mortality rate includes only deaths due to one disease and is usually reported on the basis of 100,000 persons. The population base may be defined by sex, age or other characteristics. The mortality rate must not be confused with the case fatality rate.
nosocomial infection
An infection occurring in a patient in a hospital or other health care facility and in whom it was not present or incubating at the time of admission, or the residual of an infection acquired during a previous admission. Includes infections acquired in the hospital but appearing after discharge, and also such infections among the staff of the facility.
opportunistic infection
An infection with an organism that is not normally a pathogen for an immunocompetent host, e.g., Candida albicans. Such an opportunistic infection is a sign to evaluate the patient for immunodeficiency.
prevalence rate
A quotient (rate) obtained by using as the numerator the number of persons sick or portraying a certain condition in a stated population at a particular time (point prevalence), or during a stated period of time (period prevalence), regardless of when that illness or condition began, and as the denominator the number of persons in the population in which they occurred.
reservoir
Any person, animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such manner that it can be transmitted to a susceptible host.
transmission (of an infectious agent)
- Any mechanism by which an infectious agent is spread from a source or reservoir to a person. These mechanisms are:
- Direct Transmission
- Direct and essentially immediate transfer of infectious agents to a receptive portal of entry through which human or animal infection may take place. This may be by direct contact as by touching, biting, kissing or sexual intercourse, or by the direct projection (droplet spread) of droplet spray onto the conjunctiva or onto the mucous membranes of the eye, nose or mouth during sneezing, coughing, spitting, singing or talking (usually limited to a distance of about 1 meter or less).
- Indirect Transmission
- Vehicle-Borne
- Contaminated inanimate materials or object (fomites) such as toys, handkerchiefs, soiled clothes, bedding, cooking or eating utensils, surgical instruments or dressings (indirect contact); water, food, milk, biological products including bood, serum, plasma, tissues or organs; or any substance serving as an intermediate means by which an infectious agent is transported and introduced into a susceptible host through a suitable portal of entry. The agent may or may not have multiplied or developed in or on the vehicle before being transmitted.
- Vector-Borne
- (a) Mechanical: Includes simple mechanical carriage by a crawling or flying insect through soiling of its feet or proboscis, or by passage of organisms through its gastrointestinal tract. This does not require multiplication or development of the organism.
- (b) Biological: Propagation (multiplication), cyclic development, or a combination of these (cyclopropagative) is required before the arthropod can transmit the infective form of the agent to man. An incubation period (extrinsic) is required following infection before the arthropod becomes infective. The infectious agent may be passed vertically to succeeding generations (trans-ovarian transmission); trans-stadial transmission indicates its passage from one stage of life cycle to another, as nymph to adult. Transmission may be by injection of salivary gland fluid during biting, or by regurgitation or deposition on the skin of feces or vomitusl capable of penetrating through the bite wound or through an area of trauma from scratching or rubbing. This transmission is by an infected.non-vertebrate host and not simple mechanical carriage by a vector as a vehicle. However, an arthropod in either role is termed a vector.
- (c) Airborne: The dissemination of microbial aerosols to a suitable portal of entry, usually the respiratory tract. Microbial aerosols are suspensions of partially or wholly of microorganisms. They may remain suspended in the air for long periods of time, some retaining and others losing infectivity or virulence. Particles in the 1- to 5 micrometer range are easily drawn into the alveoli of the lungs and may be retained there. Not considered as airborne are droplets and other large particles which promptly settle out (see Direct Transmission, above).
- (d) Droplet nuclei: Usually the small residues which result from evaporation of fluid from droplets emitted by an infected host (see above). They also may be created purposely by a variety of atomizing devices, or accidentally as in microbiology laboratories or in abattoirs, rendering plants or autopsy rooms. They usually remain suspended in the air for long periods of time.
- (e) Dust: The small particles of widely varying size which may arise from soil (as, for example, fungus spores separated from dry soil by wind or mechanical agitation), clothes, bedding, or contaminated floors.
zoonosis
An infection or infectious disease transmissible under natural conditions from vertebrate animals to man. May be enzootic or epizootic (see Endemic and Epidemic).
zygomycosis
The proper name for infection with members of the order Mucorales. These are opportunistic fungi with broad, generally non-septate hyphae. The most common infections are with Rhizopus spp., the common breadmold. Other members of this order of fungi known to cause human disease include: Rhizomucor spp., Mucor spp., Absidia spp., and Cuninghamella spp.