Transmission mechanisms

Which means of transmission are most important in serious infectious diseases? A breakdown of the infections causing death globally in 1995 found that infections that are transmitted from person to person (e.g. tuberculosis, measles, HIV) caused 65 percent of deaths; infections acquired from contaminated food, water, or soil (e.g. cholera and many other infections causing diarrhea, hepatitis A) caused 22 percent; vector-borne infections (primarily malaria) caused 13 percent; and infections from animals (e.g. rabies) caused 0.3 percent (WHO, 1996). These numbers are useful as general approximations, but vastly underestimate the role that animals play as a source of human pathogens. Many major human pathogens evolved in the recent or remote past from related pathogens in animals (e.g. HIV, measles, tuberculosis, malaria) (Weiss, 2001), and the majority of recently emerged infectious diseases are zoonoses, many of them viruses (Woolhouse, 2002; Smolinski et al., 2003; see also Chapter 4).

Ease of transmission varies considerably by pathogen and by site of infection. Some microbes are able to spread in multiple ways. For example, the seasonal influenza viruses can be spread by direct contact (e.g. by contaminated objects or from hands of infected person), by large or medium droplets, or by droplet nuclei (tiny-droplet aerosol) (Musher, 2003). Tuberculosis, on the other hand, is spread by droplet nuclei that are small enough to bypass the trapping mechanisms of the upper airway and can reach the lungs. Neisseria meningitidis (the cause of meningococcal meningitis and sepsis) is spread by direct contact and large or medium droplets, but not by tiny-droplet aerosol. The mechanisms of transmission and transmissibility are key characteristics of microbes that influence ease of spread and determine what interventions will be effective. Timing of onset of infectiousness relative to onset of symptoms and duration of infec-tiousness are important factors in transmission dynamics.

Environmental conditions may also influence transmissibility. Influenza virus survives better in cool temperatures and at low humidity, which may partially explain the seasonality of influenza in temperate areas. Several of the summertime outbreaks of influenza have involved transmission in air-conditioned indoor spaces, where air was cool and dried, and people were in close contact (Uyeki et al., 2003).

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