(Technical meaning of terms used in the text)

1. Carrier—A person or animal that harbors a specific infectious agent without discernible clinical disease and serves as a potential source of infection. The carrier state may exist in an individual with an infection that is inapparent throughout its course (commonly known as healthy or asymptomatic carrier), or during the incubation period, convalescence and postconvalescence of an individual with a clinically recognizable disease (commonly known as an incubatory or convalescent carrier). Under either circumstance the carrier state may be of short or long duration (temporary or transient carrier, or chronic carrier).

2. Case-fatality rate—Usually expressed as the percentage of persons diagnosed as having a specified disease who die as a result of that illness within a given period. This term is most frequently applied to a specific outbreak of acute disease in which all patients have been followed for an adequate period of time to include all attributable deaths. The case-fatality rate must be clearly differentiated from the mortality rate (q.v.). (Synonyms: fatality rate, fatality percent-age, case-fatality ratio)

3. Chemoprophylaxis—The administration of a chemical, including antibiotics, to prevent the development of an infection or the progression of an infection to active manifest disease, or to eliminate the carriage of a specific infectious agent to prevent transmission and disease in others. Chemotherapy, on the other hand, refers to use of a chemical to treat a clinically manifest disease or to limit its further progress.

4. Cleaning—The removal by scrubbing and washing, as with hot water, soap or suitable detergent or by vacuum cleaning, of infectious agents and of organic matter from surfaces on which and in which infectious agents may find favorable conditions for surviving or multiplying.

5. Communicable disease—An illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person, animal or inanimate reservoir to a susceptible host; either directly or indirectly through an intermediate plant or animal host, vector or the inanimate environment. (Synonym: infectious disease)

6. Communicable period—The time during which an infectious agent may be transferred directly or indirectly from an infected person to another person, from an infected animal to humans, or from an infected person to animals, including arthropods. 

In diseases such as diphtheria and streptococcal infection, in which mucous membranes are involved from the initial entry of the infectious agent, the period of communicability is from the date of first exposure to a source of infection until the infecting microorganism is no longer disseminated from the involved mucous membranes, i.e., from the period before the prodromata until termination of a carrier state, if the latter develops. Some diseases are more communicable during the incubation period than during the actual illness (e.g., hepatitis A, measles). 

In diseases such as tuberculosis, leprosy, syphilis, gonorrhea and some of the salmonelloses, the communicable state may exist over a long and sometimes intermittent period when active chronic lesions permit the discharge of infectious agents from the surface of the skin or through any of the body orifices.

In diseases transmitted by arthropods, such as malaria and yellow fever, the periods of communicability (or more properly infectivity) are those during which the infectious agent occurs in the blood or other tissues of the infected person in sufficient numbers to permit infection of the vector. A period of communicability (transmissibility) is also to be noted for the arthropod vector, namely, when the agent is present in the tissues of the arthropod in such form and locus (infective state) as to be transmissible.

7. Contact—A person or animal that has been in such association with an infected person or animal or a contaminated environment as to have had an opportunity to acquire the infection.

8. Contamination—The presence of an infectious agent on a body surface, in clothes, bedding, toys, surgical instruments or dressings, or other inanimate articles or substances including water and food. Pollution is distinct from contamination and implies the presence of offensive, but not necessarily infectious, matter in the environment. Contamination of a body surface does not imply a carrier state.

9. Disinfection—Killing of infectious agents outside the body by direct exposure to chemical or physical agents. High-level disinfection may kill all microorganisms with the exception of high numbers of bacterial spores; it requires extended exposure to ensure killing of most bacterial spores. It is achieved, after thorough detergent cleaning, by exposure to specific concentrations of certain disinfectants (e.g., 2% glutaraldehyde, 6% stabilized hydrogen peroxide and up to 1% peracetic acid) for at least 20 minutes. Intermediate-level disinfection does not kill spores; it can be achieved by pasteurization (750C [1670FJ for 30 minutes) or by appropriate treatment with EPA-approved disinfectants.

Concurrent disinfection is the application of disinfective measures as soon as possible after the discharge of infectious material from the body of an infected person, or after the soiling of articles with such infectious discharges; all personal contact with such discharges or articles should be minimized prior to such disinfection.

Terminal disinfection is the application of disinfective measures after the patient has been removed by death or to a hospital, or has ceased to be a source of infection, or after hospital isolation or other practices have been discontinued. Terminal disinfection is rarely practiced; terminal cleaning generally suffices (see Cleaning), along with airing and sunning of rooms, furniture and bedding. Disinfection is necessary only for diseases spread by indirect contact; steam sterilization or incineration of bedding and other items is recommended after a disease such as Lassa fever or other highly infectious diseases. 

Sterilization involves destruction of all forms of life by heat, irradiation, gas (ethylene oxide or formaldehyde) or chemical treatment.

10. Endemic—The constant presence of a disease or infectious agent within a given geographic area; it may also refer to the usual prevalence of a given disease within such area. Hyperendemic expresses a constant presence at a high level of incidence, and holoendemic a high level of prevalence with infections beginning early in life and affecting most of the population, e.g., malaria in some places.

11. Epidemic—The occurrence in a community or region of cases of an illness (or an outbreak) with a frequency clearly in excess of normal expectancy. The number of cases indicating presence of an epidemic will vary according to the infectious agent, size and type of population exposed, previous experience or lack of exposure to the disease, and time and place of occurrence; epidemicity is thus relative to usual frequency of the disease in the same area, among the specified population, at the same season of the year. A single case of a communicable disease long absent from a population or the first invasion by a disease not previously recognized in that area requires immediate reporting and epidemiologic investigation; two cases of such a disease associated in time and place are sufficient evidence of transmission to be considered an epidemic. (See Report of a Disease)

12. Host—A person or other living animal, including birds and arthropods, that affords subsistence or lodgment to an infectious agent under natural (as opposed to experimental) conditions. Some protozoa and helminths pass successive stages in alternate hosts of different species. Hosts in which the parasite attains maturity or passes its sexual stage are primary or definitive hosts; those in which the parasite is in a larval or asexual state are secondary or intermediate hosts. A transport host is a carrier in which the organism remains alive but does not undergo development.

13. Immune individual—A person or animal that has specific protective antibodies and/or cellular immunity as a result of previous infection or immunization, or is so conditioned by such previous specific experience as to respond in such a way that prevents the development of infection and/or clinical illness following re-exposure to the specific infectious agent. Immunity is relative: a level of protection that could be adequate under ordinary conditions may be overwhelmed by an excessive dose of the infectious agent or by exposure through an unusual portal of entry; protection may also be impaired by immunosuppressive drug therapy, concurrent disease or the aging process. (See Resistance.)

14. Immunity—That resistance usually associated with the presence of antibodies or cells having a specific action on the microorganism concerned with a particular infectious disease or on its toxin. Effective immunity includes both cellular immunity, which is conferred by T-lymphocyte sensitization, and/or humoral immunity, which is based on B-lymphocyte response. Passive immunity is attained either naturally by transplacental transfer from the mother, or artificially by inoculation of specific protective antibodies (from immunized animals, or convalescent hyperimmune serum or immune serum globulin [human]); it is of short duration (days to months). Active humoral Immunity, which usually lasts for years, is attained either naturally by infection with or without clinical manifestations, or artificially by inoculation of the agent itself in killed, modified or variant form, or of fractions or products of the agent.

15. Inapparent infection—The presence of infection in a host without recognizable clinical signs or symptoms. Inapparent infections are identifiable only by laboratory means such as a blood test or by the development of positive reactivity to specific skin tests. (Synonyms: asymptomatic, subclinical, occult infection)

16. Incidence rate—The number of new cases of a specified disease diagnosed or reported during a defined period of time, divided by the number of persons in a stated population in which the cases occurred. This is usually expressed as cases per 1,000 or 100,000 per annum. This rate may be expressed as age- or gender-specific or as specific for any other population characteristic or subdivision. (See Morbidity rate and Prevalence rate.) Attack rate, or case rate, is a proportion measuring cumulative incidence often used for particular groups, observed for limited periods and under special circumstances, as in an epidemic; it is usually expressed as percent (cases per 100 in the group). The secondary attack rate is 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 exposed contacts; the denominator may be restricted to susceptible contacts when determinable. Infection rate is a proportion that expresses the incidence of all identified infections, manifest and inapparent.

17. Incubation period—The time interval between initial contact with an infectious agent and the first appearance of symptoms associated with the infection. In a vector, it is the time between entrance of an organism into the vector and the time when that vector can transmit the infection (extrinsic incubation period). The period in people between the time of exposure to a parasite and the time when the parasite can be detected in blood or stool is called the prepatent period.

18. Infected Individual—A person or animal that harbors an infectious agent and who has either manifest disease (see Patient or sick person) or inapparent infection (see Carrier). An infectious person or animal is one from whom the infectious agent can be naturally acquired.

19. Infection—The entry and development (of many parasites) or multiplication of an infectious agent in the body of persons or animals. Infection is not synonymous with infectious disease; the result may be inapparent (see Inapparent infection) or manifest (see Infectious disease). The presence of living infectious agents on exterior surfaces of the body, or on articles of apparel or soiled articles, is not infection, but represents contamination of such surfaces and articles. (See Infestation and Contamination.)

20. Infectious agent—An organism (virus, rickettsia, bacteria, fuingus, protozoan or helminth) that is capable of producing infection or infectious disease. Infectivity expresses the ability of the disease agent to enter, survive and multiply in the host; infectiousness indicates the relative ease with which a disease is transmitted to other hosts.

21. Infectious disease—A clinically manifest disease of humans or animals resulting from an infection. (See Infection.)

22. Infestation—For persons or animals, the lodgment, development and reproduction of arthropods on the surface of the body or in the clothing. Infested articles or premises are those that harbor or give shelter to animal forms, especially arthropods and rodents.

23. Insecticide—Any chemical substance used for the destruction of insects, whether applied as powder, liquid, atomized liquid, aerosol or “paint” spray; residual action is usual. The term larvicide is generally used to designate insecticides applied specifically for destruction of immature stages of arthropods; adulticide or imagocide, to designate those applied to destroy mature or adult forms. The term insecticide is often used broadly to encompass substances for the destruction of all arthropods, but acaricide is more properly used for agents against ticks and mites. More specific terms such as lousicide and miticide are sometimes used.

24. Isolation—As applied to patients, isolation represents separation, for the period of communicability, of infected persons or animals from others in such places and under such conditions as to prevent or limit the direct or indirect transmission of the infectious agent from those infected to those who are susceptible to infection or who may spread the agent to others. In contrast, quarantine (q.v.) applies to restrictions on the healthy contacts of an infectious case. CDC has recommended that Universal Precautions be used consistently for all patients (in hospital settings as well as outpatient settings) regardless of their bloodborne infection status. This practice is based on the possibility that blood and certain body fluids (any body secretion that is obviously bloody, semen, vaginal secretions, tissue, CSF, and synovial, pleural, peritoneal, pericardial and amniotic fluids) of all patients are potentially infectious for HIV, HBV and other bloodborne pathogens. Universal precautions are intended to prevent parenteral, mucous membrane and nonintact skin exposures of healthcare workers to bloodbome pathogens. Protective barriers include gloves, gowns, masks and protective eyewear or face shields. A private room is indicated if patient hygiene is poor. Waste management is controlled by local and state authority. 

There are two basic requirements that are common for care of all potentially infectious cases:

• Hands must be washed after contact with the patient or potentially contaminated articles and before taking care of another patient; and

• Articles contaminated with infectious material should be appropriately discarded or bagged and labeled before being sent for decontamination and reprocessing.

Recommendations made for isolation of cases in section 9B2 of each disease may allude to the methods that had been recommended by CDC (CDC Guideline for Isolation Precautions in Hospitals) as category-specific isolation precautions, in addition to universal precautions, based on the mode of transmission of the specific disease. These categories are as follows:

1) Strict isolation: This category is designed to prevent transmission of highly contagious or virulent infections that may be spread by both air and contact. The specifications, in addition to those above, include a private room and the use of masks, gowns and gloves for all persons entering the room. Special ventilation requirements with the room at negative pressure to surrounding areas are desirable.

2) Contact isolation: For less highly transmissible or serious infections, for diseases or conditions that are spread primarily by close or direct contact. In addition to the basic requirements, a private room is indicated, but patients infected with the same pathogen may share a room. Masks are indicated for those who come close to the patient, gowns are indicated if soiling is likely and gloves are indicated for touching infectious material. 

3) Respiratory isolation: To prevent transmission of infectious diseases over short distances through the air, a private room is indicated, but patients infected with the same organism may share a room. In addition to the basic requirements, masks are indicated for those who come in close contact with the patient; gowns and gloves are not indicated.

4) Tuberculosis isolation (AFB isolation): For patients with pulmonary tuberculosis who have a positive sputum smear or a chest x-ray that strongly suggests active tuberculosis. Specifications include use of a private room with special ventilation and closed door. In addition to the basic requirements, respirator-type masks are used by those entering the room. Gowns are used to prevent gross contamination of clothing. Gloves are not indicated.

5) Enteric precautions: For infections transmitted by direct or indirect contact with feces. In addition to the basic requirements, specifications include use of a private room if patient hygiene is poor. Masks are not indicated; gowns should be used if soiling is likely and gloves are to be used for touching contaminated materials.

6) Drainage/secretion precautions: To prevent infections transmitted by direct or indirect contact with purulent material or drainage from an infected body site. A private room and masking are not indicated; in addition to the basic requirements, gowns should be used if soiling is likely and gloves should be used for touching contaminated materials.

25. Morbidity rate—An incidence rate (q.v.) 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 gender or age group, or to those with certain other characteristics.

26. Mortality rate—A rate calculated in the same way as an incidence rate (q.v.), by dividing the number of deaths occurring in the population during the stated period of time, usually a year, by the number of persons at risk of dying during the period. A total or crude mortality rate utilizes deaths from all causes, usually expressed as deaths per 1,000. A disease-specific mortality rate covers deaths due to only one disease and is often reported on the basis of 100,000 persons. The population base may be defined by gender, age or other characteristics. The mortality rate must not be confused with case-fatality rate (q.v.). (Synonym: death rate)

27. Nosocomial infectionAn infection occurring in a patient in a hospital or other healthcare facility 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. (Synonym: hospital-acquired infection)

28. Pathogenicity—The property of an infectious agent that determines the extent to which overt disease is produced in an infected population, or the power of an organism to produce disease.

29. Prevalence rate—The total 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, divided by the population at risk of having the disease or condition at the point in time or midway through the period in which they occurred.

30. QuarantIne—Restriction of the activities of well persons or animals who have been exposed to a case of communicable disease during its period of communicability (i.e., contacts) to prevent disease transmission during the incubation period if infection should occur.

1) Absolute or complete quarantine: The limitation of freedom of movement of those exposed to a communicable disease for a period of time not longer than the longest usual incubation period of that disease, in such manner as to prevent effective contact with those not so exposed. (See Isolation.)

2) Modified quarantine: A selective, partial limitation of freedom of movement of contacts, commonly on the basis of known or presumed differences in susceptibility and related to the danger of disease transmission. It may be designed to accommodate particular situations. Examples are exclusion of children from school, exemption of immune persons from provisions applicable to susceptible persons, or restriction of military populations to the post or to quarters. It includes: personal surveillance, the practice of close medical or other supervision of contacts to permit prompt recognition of infection or illness but without restricting their movements; and segregation, the separation of some part of a group of persons or domestic animals from the others for special consideration, control or observation; removal of susceptible children to homes of immune persons; or establishment of a sanitary boundary to protect uninfected from infected portions of a population.

31. Report of a disease—An official report notifying an appropriate authority of the occurrence of a specified communicable or other disease in humans or in animals. Diseases in humans are reported to the local health authority; those in animals, to the livestock, sanitary, veterinary or agriculture authority. Some few diseases in animals, also transmissible to humans, are reportable to both authorities. Each health jurisdiction declares a list of reportable diseases appropriate to its particular needs (see Communicable Disease Reporting). Reports should also list suspected cases of diseases of particular public health importance, ordinarily those requiring epidemiologic investigation or initiation of special control measures.

When a person is infected in one health jurisdiction and the case is reported from another, the health authority receiving the report should notify the jurisdiction where infection presumably occurred, especially when the disease requires examination of contacts for infection, or if food, water or other common vehicles of infection may be involved.

In addition to routine report of cases of specified diseases, special notification is required of all epidemics or outbreaks of disease, including diseases not listed as reportable. (See Epidemic.)

32. Reservoir (of infectious agents)—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.

33. Resistance—The sum total of body mechanisms that interpose barriers to the invasion or multiplication of infectious agents, or to damage by their toxic products. Inherent resistance—an ability to resist disease independent of immunity or of specifically developed tissue responses; it commonly resides in anatomic or physiologic characteristics of the host and may be genetic or acquired, permanent or temporary. (See Immunity.) (Synonym: Nonspecific immunity)

34. Source of infection—The person, animal, object or substance from which an infectious agent passes to a host. Source of infection should be clearly distinguished from source of contamination, such as overflow of a septic tank contaminating a water supply, or an infected cook contaminating a salad. (See Reservoir.) 

35. Surveillance of disease—As distinct from surveillance of persons (see Quarantine, 2), surveillance of disease is the continuing scrutiny of all aspects of occurrence and spread of a disease that are pertinent to effective control. Included are the systematic collection and evaluation of 

1) morbidity and mortality reports;

2) special reports of field investigations of epidemics and of individual cases;

3) isolation and identification of infectious agents by laboratories;

4) data concerning the availability, use and untoward effects of vaccines and toxoids, immune globulins, insecticides and other substances used in control;

5) information regarding immunity levels in segments of the population; and

6) other relevant epidemiologic data. A report summarizing the above data should be prepared and distributed to all cooperating persons and others with a need to know the results of the surveillance activities. The procedure applies to all jurisdictional levels of public health from local to international. Serologic surveillance identifies patterns of current and past infection using serologic tests.

36. Susceptible—A person or animal not possessing sufficient resistance against a particular pathogenic agent to prevent contracting infection or disease when exposed to the agent.

37. Suspect—In infectious disease control, illness in a person whose history and symptoms suggest that he or she may have or be developing a communicable disease.

38. Transmission of infectious agents—Any mechanism by which an infectious agent is spread from a source or reservoir to a person. These mechanisms are as follows:

1) 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 such as 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 m or less).

2) Indirect transmission:

a) Vehicle-borne—Contaminated inanimate materials or objects (fomites) such as toys, handkerchiefs, soiled clothes, bedding, cooking or eating utensils, surgical instruments or dressings; water, food, milk, and biological products including blood, 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.

b) Vector-borne—(i) 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. (ii) 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 humans. An incubation period (extrinsic) is required following infection before the arthropod becomes infective. The infectious agent may be passed vertically to succeeding generations (transovarian transmission); transstadial 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 other material capable of penetrating through the bite wound or through an area of trauma from scratching or rubbing. This transmission is by an infected nonvertebrate host and not simple mechanical carriage by a vector as a vehicle. However, an arthropod in either role is termed a vector

3) Airborne: The dissemination of microbial aerosols to a suitable portal of entry, usually the respiratory tract. Microbial aerosols are suspensions of particles in the air consisting 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-pm 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 that promptly settle out (see Direct transmission, above).

a) Droplet nuclei—Usually the small residues that result from evaporation of fluid from droplets emitted by an infected host (see above). They may also 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.

b) Dust—The small particles of widely varying size that may arise from soil (as, e.g., fungus spores separated from dry soil by wind or mechanical agitation), clothes, bedding or contaminated floors. 

39. Virulence—The degree of pathogenicity of an infectious agent, indicated by case-fatality rates and/or the ability of the agent to invade and damage tissues of the host.

40. ZoonosisAn infection or infectious disease transmissible under natural conditions from vertebrate animals to humans.  May be enzootic or epizootic (see Endemic and Epidemic)