Introduction to Infection Prevention
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 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.

Respiratory transmission associated with the inhalation of droplets, i.e., airborne particles of moisture greater than 5 µm that may contain potentially infectious pathogens, 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.¹

Airborne transmission is a form of respiratory transmission associated with inhalation of droplet nuclei, i.e., residuals of droplets ranging in size from 1-5 µm 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, i.e., extend beyond 3 feet of the source. Droplet nuclei may also contaminate intermediate objects or environmental surfaces.