>Heterogeneity increases survivability of the *species*, but does little >to protect the individual. What you're not taking into account is contagion. Amongst a homogeneous population, a pathogen that infects your friends can likely infect you. Amongst a heterogeneous population, if the same pathogen infects a friend, there's a significantly lower probability it can infect you. Now, if you're promiscuous and come into contact with enough strangers, you'll catch the pathogen either way. But if you're not promiscuous, you greatly reduce the change of contracting the pathogen if you are part of a heterogeneous population. How does this affect networks? Well, if you're a webserver or mailserver that talks to everyone, the heterogeneity doesn't buy you so much (other than, as you said, there might be more pathogens for popular systems). But if you're configured to not talk to the whole world (via a firewall, or something equivalent), then you're a whole lot safer if the machines you do communicate with are different from you in ways that make contagion harder. Cheers, Mark
