An elementary model of sociogeographic network structure in an urban minority community suggests externally applied stress, particularly that which triggers frequent individual or family moves of increasing distance, may cause a sudden 'phase change' resulting in disconnection of previously integrated subgroups from the community. Such 'community meltdown' would seriously disrupt mechanisms for social control, economic opportunity and socialization of youth, while intensifying substance abuse and indiscriminate and frequent sexual activity, particularly among the very young. 'Phase change' of this nature would seem to preclude success of programs to control spread of HIV infection, particularly in heterosexual populations. The possibility is explored that programs of 'community recrystalization' in disintegrated urban area might likewise need to exceed some threshold of investment and organizing activity before becoming effective. However, if supported to levels above threshold, this work implies such programs might have very great impact in a very short time. The possibility of interaction between behavioral pathologies resulting from the initial meltdown and further consequent deterioration in sociogeographic network structure leads to speculation that the threshold for recrystalization may become significantly and progressively greater than for meltdown. Implications of these matters for diffusion of HIV into the general population are discussed.