Genetic robustness in the genetic networks of influenza A virus might have an environmental robustness correlate. Influenza A virus is notorious for its characteristically broad host range, including various bird species and mammals such as domesticated pigs whose proximity to humans may spur pandemics. [59] [60] [61] As noted earlier, relatively broad host range of a pathogen may be defined as environmental robustness. The ability for influenza A viruses to traverse the aforementioned neutral network of genetic robustness may have an important environmental robustness component. The virus may be able to easily create a robust neutral network of genotypes associated with attachment/entry of various host cells due to intrinsic environmental robustness that evolved via selection to infect a variety of host environments. In influenza A virus, whether environmental robustness preceded genetic robustness, or vice versa, is difficult to know. But environmental robustness of viruses such as influenza A virus undoubtedly fosters the ability to reside in various host species that may serve as reservoirs that allow occasional spillover into a species of interest such as humans. 52 Robustness may also be relevant in explaining aspects of the current AIDS pandemic. A global increase in robustness has been invoked to explain an apparent global decline in the virulence of HIV-1 strains. 38 Although it was not discussed extensively, the robustness concept in this example seems to be environmentally, not mutationally, determined. The argument is that a perceived decline in HIV-1 virulence might be due to generally evolved increase in environmental robustness of the virus, owing to its worldwide spread. The virus has thus encountered variable subpopulations of humans and has undergone selection for environmental robustness, which coincides with improved performance across environments at the expense of reduced reproduction ͑i.e., virulence͒ on average. This explanation assumes that the stated tradeoff in viral traits is a generalized phenomenon, and that results for increased genetic robustness at the expense of reduced reproduction 9,13 apply in the case of environmental robustness as well.