Simply saying that the virulence of a pathogen within the population goes up when population density goes up, down when resistance goes up. By population resistance I refer to a combination of factors such as pre-existing immunity (ie through vaccination), innate immune status, general health and nutrition level, hygeine habits (ie frequency of handwashing), food preparation habits, agricultural practices, etc. etc. It is the inverse of susceptibility.
This law is built on the fact variants with a capcity for lethality have always existed within the various quaspecies of pathogens continuously circulating within human and animal populations.
However, since infectious spread is severly compromised by high virulence, such strains can take hold when infectivity is saturated in extremely densely populated groups with high susceptibility. Thus the emergent "killer strains" everyone talks about in the news are not some new creation from thin air, but normally latent strains whose prevalence is increased by changes in human behaviour that increase population density and/or decrease resistance. Unfortunately these two factors are often coupled. For example poverty which decreases resistance through its effects on nutrition and health, tends to be prevalent in overly populated cities.
So don't be afraid of H5N1 or any other pestilence that might be cast down upon our poor helpless souls by the Almighty. Instead, fear the dark side of irresponsible civilizations: poverty, malnutrition, inadequate health care and lack of education within our massive, rapidly growing, and highly interactive global population.