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Complement control protein, required for EEV, homologue to C3L

Source: Adapted from Fields et al. (1996, Table 2, p. 2643) with additional information from Goebel et al. (1990). aORFs are named and color coded according to the restriction map show in Fig. 6.2.

Source: Adapted from Fields et al. (1996, Table 2, p. 2643) with additional information from Goebel et al. (1990). aORFs are named and color coded according to the restriction map show in Fig. 6.2.

defenses. It is clear that these various viral functions are required for successful viral infection of their hosts in nature, and the existence of these viral activities has been very useful for our understanding of host defenses against viral infection. We will return to this topic in Chapter 8.

Genus Orthopoxvirus

The best known poxviruses are the orthopoxviruses. Vaccinia virus has been widely studied in the laboratory as a model for the replication of poxviruses and has been used to immunize hundreds of millions of people against smallpox virus, also known as variola (from the Latin word for "spotted"). The extensive knowledge of vaccinia virus gained from laboratory and clinical studies has also led to its use as a vector to express foreign antigens in cultured cells or in animals (Chapter 9). Other members of the genus infect a variety of domestic and wild animals (Table 6.3). A similarity tree that illustrates the relationships among the orthopoxviruses is shown in Fig. 6.4. Notice that the human smallpox viruses, variola major and its close relative variola minor, are related to cowpox virus, but the lineage formed by these viruses is distinct from the lineage that contains vaccinia virus and monkeypox virus. Nonetheless, all of these viruses are cross-protective.

Smallpox Disease

Smallpox once caused vast epidemics in human populations. It was already endemic in India 2000 years ago and had spread to China, Japan, Europe, and northern Africa by 700 a.d. It was introduced into the New World by the Europeans during their explorations. Infection resulted in a fatality rate of 20-30% in most populations and at one time the virus infected virtually the entire population of Europe. Thus, the virus was responsible for a significant fraction of all human deaths on the Continent. When introduced into virgin populations in the New World, the mortality rate was much higher, probably due in part to a lack of previous selection for resistance to smallpox and in part due to the breakdown of the social system caused by the simultaneous infection of most of the population. The importance of smallpox, measles, and other Old World plagues in the conquest and settlement of the Americas by the Europeans was discussed in Chapter 4. A detailed description of the effect of smallpox virus on human civilization through the ages can be found in the book Princes and Peasants: Smallpox in History by Hopkins. The history of nations has often been changed by the early death of rulers from smallpox or from the appearance of smallpox in armies. As one example of interest to Americans, an invasion of Canada by American

Family Poxviridae

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