Читать книгу Principles of Virology, Volume 2 - Jane Flint, S. Jane Flint - Страница 95

Respiratory transmission depends on the density of virus particles in the secretion, and the duration of a liquid droplet in the air. Aerosols are produced during speaking and normal breathing, while coughing produces even more forceful expulsion. Transmission from the nasal cavity is facilitated by sneezing and is much more effective if infection induces the production of nasal secretions. A sneeze from an influenza virus-infected individual produces up to 20,000 droplets (in contrast to several hundred expelled by coughing), although a recent publication assessing influenza virus transmission found almost no virus in a “sneeze vapor”; virtually all infectious virus was found in the liquids expelled during a cough. As noted when we discussed viral entry, the size of a droplet affects its “hang time”: large droplets fall to the ground, but smaller droplets (1 to 4 μm in diameter) may remain suspended in the air for many hours. Such particles may not only come in contact with a naïve host but also penetrate into the lower respiratory tract. Nasal and oral secretions also frequently contaminate hands or tissues. The infection may be transmitted when these objects contact another person’s fingers, and that person in turn touches his or her nose or conjunctiva. In today’s crowded society, the physical proximity of people may select for viruses that spread efficiently by this route. Sneezing and coughing may be the body’s way of trying to eliminate an irritant in the respiratory tract. While it is appealing to speculate that viruses may have been selected that induce thunderous noises and ejection of alarming volumes of body fluids to ensure transmission to new hosts, this hypothesis has not yet been proven (Box 2.12).