This study is a large, joint project of several medical and scientific institutions in Essen, Berlin, Hamburg, Frankfurt, Munich, Bochum, and Cologne. The purpose of this study is to investigate the incidence of human immunodeficiency virus (HIV), hepatitis and other sexually transmitted diseases such as chlamydia, gonorrhea, and syphilis in people at risk of infection. With the exception of HIV infection, most of these sexually transmitted diseases are curable. However, if they are not diagnosed in time, they too can lead to serious health problems. This study can help to develop better strategies to prevent such infections in the future. During the study visits, participants are asked to complete a questionnaire about their HIV risk behavior and submit blood, urine, and swab specimens that will be tested for sexually transmitted diseases. As part of this collaborative effort, we are also developing vaccines against HIV that we would like to test in future clinical trials. Therefore, the participants in the study are asked to complete another questionnaire to find out if they would participate in such a study and if such a study would be possible in the future.