How to succeed as a virus: strategies for dealing with the immune system

Viruses may be viewed as genetic information whose success depends on avoid ing elimination from individual hosts, or, if this is not possible, in pers isting in the population of their hosts. The immune system represents the c rucial defense mechanism responsible for the elimination of viruses from in dividual hosts and for the establishment of immunity that prevents a recurr ing infection by the same virus. Herd immunity, i.e., immunity of the popul ation against infection resulting from the immunity of a certain fraction o f the individuals of the population, represents an important concept in the interaction of viruses with their hosts. Thus, if the number of susceptibl e hosts decreases below a critical threshold, viruses may risk extinction b ecause they literally run out of substrate. This possibility is increased d ue to the viruses' low resistance to inactivation outside their hosts by ph ysical influences, such as heat and ultraviolet radiation. Some viruses hav e adopted a strategy of dual host tropism, i.e., they may reside in reservo ir hosts that permit them to survive for extended periods of times. Example s of such viruses are the large and taxonomically diverse group of arboviru ses. Moreover, although not normally discussed under this aspect, influenza viruses can also be said to have adopted this strategy, in view of water f owl representing reservoir hosts from which complete viruses may directly c ross over to mammals, as was the case with the equine Jilin (Guo et al., 19 95) or, more recently, the H5 subtype of influenza virus in humans (Shortri dge et al., 1998). In addition, influenza viruses of birds may be transmitt ed, albeit only partially through genetic reassortment (Shu et al., 1996). (C) 1999 Elsevier Science B.V. All rights reserved.