Indirect Person-to Person Transmission

Transmission Via Fomites

Fomites are inanimate objects that can become contaminated with infectious agents and serve as a mechanism for transfer between hosts. The classic example of a fomite is a park water fountain from which many people drink. Infectious agents deposited by one person can potentially be transmitted to a subsequent drinker. However, many objects that we come into contact with can serve as fomites: doorknobs, elevator buttons, hand rails, phones, writing implements, keyboards, toys in a day care center, etc. Even a stethoscope can serve as a fomite if it isn't cleansed. A 2009 study reported MRSA (methicillin-resistant Staphylococcus aureus) contamination on 15% of stethoscopes tested. Even clothing from healthcare personnel can be contaminated. Needless to say, hand washing, respiratory hygiene and period cleansing of potential fomites would substantially reduce diarrheal diseases, respiratory diseases, and soft tissue infections (e.g., impetigo and MRSA-related skin and soft tissue infections.

For detailed information on fomites, see Boone SA et al: Significance of Fomites in the Spread of Respiratory and Enteric Viral Disease. Appl Environ Microbiol. 2007 March; 73(6):1687-1696.

Respiratory Transmission

Many diseases can be spread via respiratory droplets that are propelled into the air by sneezing and coughing. The aerosols generated by coughing and sneezing contain droplets of moisture of varying size that are contaminated with infectious agents. Droplets can be propelled up to 6-12 feet depending on the size of the droplets and the force of expulsion. The largest droplets are more likely to fall to the floor or ground fairly rapidly. Medium sized droplets are more likely to enter the nasopharynx of someone nearby, and can adhere to nasopharyngeal epithelial cells where they can cause infection. Moisture in the smallest particles tends to evaporate rapidly, resulting in the formation of so-called droplet nuclei, which are very light and can remain airborne for some time. Droplet nuclei can travel from room to room or through ventilation ducts; they can also travel around the edges of standard paper masks that are typically used during surgery or other medical procedures. Because of their small size and light weight, their movement is dictated by air currents, and, if inhaled, they can flow with inhaled air far down the respiratory tract, possibly reaching the alveoli. As a result, inhaling large or medium sized droplets tends to cause upper respiratory tract infections, while inhaling the smallest particles (droplet nuclei) can cause pneumonia. Diseases that are typically spread by inhalation of medium sized droplets include bacteria (e.g., Neisseria meningitidis [a cause of bacterial meningitis] and Streptococcus) and viruses (e.g., many viruses causing the common cold, laryngitis, tracheitis and also influenza viruses). In contrast, tuberculosis (caused by the bacterium Mycobacterium tuberculosis is spread by droplet nuclei.

Respiratory droplets can also contaminate inanimate objects (fomites) via coughing or sneezing or by transform from hands contaminated with a sneeze or cough. A susceptible individual can therefore acquire disease by inhalation of airborne particles. Alternatively, one can become infected by touching a person or inanimate object that is contaminated and then rubbing one's eyes or allowing the infectious particles to enter the nose or mouth. Needless to say, hand washing and respiratory hygiene provide a simple way of limiting this mode of spread. For a detailed summary of this topic, see:

For an interesting perspective on respiratory transmission of infectious diseases see Musher, DM: How contagious are common respiratory infections? N Engl J Med 2003;348:1256-66.

Mycobacterium tuberculosis

CDC Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Facilities, 1994

(The entire document can be found at the following link.)


"M. tuberculosis is carried in airborne particles, or droplet nuclei, that can be generated when persons who have pulmonary or laryngeal TB sneeze, cough, speak, or sing (6). The particles are an estimated 1-5 um in size, and normal air currents can keep them airborne for prolonged time periods and spread them throughout a room or building (7). Infection occurs when a susceptible person inhales droplet nuclei containing M. tuberculosis, and these droplet nuclei traverse the mouth or nasal passages, upper respiratory tract, and bronchi to reach the alveoli of the lungs. Once in the alveoli, the organisms are taken up by alveolar macrophages and spread throughout the body. Usually within 2-10 weeks after initial infection with M. tuberculosis, the immune response limits further multiplication and spread of the tubercle bacilli; however, some of the bacilli remain dormant and viable for many years. This condition is referred to as latent TB infection. Persons with latent TB infection usually have positive purified protein derivative (PPD)-tuberculin skin-test results, but they do not have symptoms of active TB, and they are not infectious.

In general, persons who become infected with M. tuberculosis have approximately a 10% risk for developing active TB during their lifetimes. This risk is greatest during the first 2 years after infection. Immunocompromised persons have a greater risk for the progression of latent TB infection to active TB disease; HIV infection is the strongest known risk factor for this progression. Persons with latent TB infection who become coinfected with HIV have approximately an 8%-10% risk per year for developing active TB (8). HIV-infected persons who are already severely immunosuppressed and who become newly infected with M. tuberculosis have an even greater risk for developing active TB (9-12).

The probability that a person who is exposed to M. tuberculosis will become infected depends primarily on the concentration of infectious droplet nuclei in the air and the duration of exposure. Characteristics of the TB patient that enhance transmission include a) disease in the lungs, airways, or larynx; b) presence of cough or other forceful expiratory measures; c) presence of acid-fast bacilli (AFB) in the sputum; d) failure of the patient to cover the mouth and nose when coughing or sneezing; e) presence of cavitation on chest radiograph; f) inappropriate or short duration of chemotherapy; and g) administration of procedures that can induce coughing or cause aerosolization of M. tuberculosis (e.g., sputum induction). Environmental factors that enhance the likelihood of transmission include a) exposure in relatively small, enclosed spaces; b) inadequate local or general ventilation that results in insufficient dilution and/or removal of infectious droplet nuclei; and c) recirculation of air containing infectious droplet nuclei. Characteristics of the persons exposed to M. tuberculosis that may affect the risk for becoming infected are not as well defined. In general, persons who have been infected previously with M. tuberculosis may be less susceptible to subsequent infection. However, reinfection can occur among previously infected persons, especially if they are severely immunocompromised. Vaccination with Bacille of Calmette and Guerin (BCG) probably does not affect the risk for infection; rather, it decreases the risk for progressing from latent TB infection to active TB (13). Finally, although it is well established that HIV infection increases the likelihood of progressing from latent TB infection to active TB, it is unknown whether HIV infection increases the risk for becoming infected if exposed to M. tuberculosis."