The research proposed here will focus on two critically important problems facing American society, the adverse personal and societal consequences of illicit drug abuse and the introduction, reintroduction, and spread of human pathogens. The study, Social Network Characteristics of a Highly Active HIV/STD Core Group, builds on our work on the characteristics, drug use, and HIV risk behaviors of male prostitutes and their sexual and drug using partners in Houston, Texas. Because of high levels of drug abuse, risky sexual activity and undiagnosed or untreated HIV/STDs, male prostitutes appear to play a significant role in periodically reintroducing sexually transmitted diseases into sexually active networks. Our research also indicates that male prostitutes travel frequently and widely, suggesting that these men may serve as 'vectors' for disease reintroduction into low prevalence, geographically disparate populations. In essence, male prostitutes may provide bridges between populations that otherwise are socially and/or geographically isolated in terms of the epidemiologically relevant links required to transmit pathogens. The principal aims of this study are to examine drug abuse in male prostitutes, properties of their drug use, sexual, and social networks, and the extent to which these are associated with the emergence and re- emergence of disease in previously unaffected populations. Secondly, the study aims to develop a protocol that will be used to study disease introduction and reintroduction in other groups involved in drug abuse. We propose to accomplish these aims by examining the characteristics of and change within a core network of drug using male prostitutes. The design of the study is a longitudinal network analysis. We will collect sociodemographic, drug use, sexual behavior, and personal network linkage data on participants who we believe may be embedded in larger networks, perhaps as high as 15,000 persons overall. We will initially sample the networks of 25 focal male prostitutes and their social, drug use, and sexual links and the drug use, sexual, and social links of the secondary networks. Whereas research to date has stressed personal networks, the 'building blocks' of larger social networks, our study will also collect data on structural patterns formed by the interconnection of personal networks into large social networks.