Infection is the invasion and multiplication of microorganisms in body tissues resulting in local cellular injury as a consequence of competitive metabolism, toxin production, and immune-mediated reactions. The “chain of infectiont,” i.e., the transmission of pathogenic organisms in any setting requires three elements: (1) a source or reservoir of infectious agents, (2) a susceptible host with a portal of entry receptive of the agent, and (3) a mode of transmission for the agent.1
Sources of Infectious Agents
Pathogens associated with HAIs are derived primarily from human sources, but contaminated objects and environmental sources are also implicated.1 Human sources include patients, HCP, house hold members, and visitors. A source-individual may have an acute infection, or may be transiently or chronically colonized by pathogenic organisms. It is also important to recognize that the source-individual may be asymptomatic or may be in the incubation (subclinical) phase of infection.
Infection is the result of complex interactions between host and pathogenic organisms. While the numbers, pathogenicity, virulence, and antigenicity of organisms are important determinants; the establishment of infection and its severity relate to the state of host defense mechanisms.1 Some susceptible hosts become colonized but remain asymptomatic. Others progress from colonization to symptomatic disease, either immediately or following a period of asymptomatic latency.
Modes of Transmission
Pathogens may be transferred from the source to a host by direct or indirect contact transmission and by respiratory transmission. Respiratory transmission may result from inhalation of droplets; or from inhalation of droplet nuclei, i.e., airborne transmission.1 Droplets and droplet nuclei are generated when people talk, breath, cough, or sneeze; or when water is converted to a fine mist by medical/dental devices, such as high-speed handpieces, ultrasonic instruments, or by lasers and electrosurgical units.1
Direct contact transmission occurs when pathogens are transferred between individuals without a contaminated intermediate person, object, or environmental surface.1 For example, when blood or other potentially infectious materials (OPIM) from an infected person enters the body of a susceptible person through direct contact with mucous membrane or breaks in the skin, e.g., when pathogens are transferred from a patient to a HCP during ungloved contact with mucous membrane or skin.
Indirect contact transmission occurs when pathogens are transferred between individuals via a contaminated intermediate person, object, or environmental surface.1 For example, when the hands of HCP become contaminated and hand hygiene is not performed prior to touching the next patient; when contaminated patient-care items are shared between patients without having been adequately cleaned, disinfected, or sterilized; or in association with contaminated sharps and needlestick injuries.
Droplets are airborne particles of moisture greater than 5 µm, which may contain potentially infectious pathogens.1 Respiratory transmission associated with droplet inhalation is generally limited to within 3 feet of the source; but it may also result from physical transfer of pathogens from a body surface, such as the hands contaminated with respiratory secretions; or contact of a susceptible host with contaminated intermediate objects or environmental surfaces.
Droplet nuclei, ranging in size from 1-5 µm, are residuals of droplets that while suspended in air dried out, but may still contain potentially infectious pathogens.1 In a cool setting, droplet nuclei may remain in the air indefinitely and travel long distances. The risk of respiratory transmission associated with inhalation of droplet nuclei, defined as airborne transmission, extends beyond 3 feet of the source. Droplet nuclei may also contaminate intermediate objects or environmental surfaces.