Читать книгу Principles of Virology, Volume 2 - Jane Flint, S. Jane Flint - Страница 94

Viruses that cannot spread from host to host face extinction. Just as there are many ways that viruses can infect host species, there are even more ways for them to get out. The release of virus particles from an infected host is called shedding. While most transmission events are attributable to such release, there are some exceptions. These exceptions include vertical transmission of integrated viral genomes in the host germ line from mother to child, and transmission via blood transfusions or organ transplantation, such as can occur with human immunodeficiency virus type 1 and hepatitis viruses.

During localized infections in or near one of the body openings, shedding can occur from the primary site of virus reproduction. The papillomaviruses cause genital warts; these viruses reproduce locally in the genital epithelium and are transmitted to hosts via sexual contact. In contrast, virus particles that result in disseminated infections can exit the host from many sites. Effective transmission of virus particles from one host to another depends on the concentration of released particles and the mechanisms by which the virus particles are introduced into the next host. The shedding of small quantities of virions may be insufficient to cause new infections, while the shedding of high concentrations may facilitate transmission via minute quantities of tissue or body fluid. For example, the concentration of hepatitis B virus particles in blood can be so high that a few microliters is sufficient to initiate an infection. Similarly, when you receive a mosquito “bite” (actually a sting), a tiny amount (less than a microliter) of its saliva is injected, which may contain sufficient particles to result in widespread infection. An interesting feature of such infections is that the mosquito saliva also numbs the site, stops the blood from clotting, and induces inflammation, which can help to spread the infection.