Figure 2. HIV prevalence in participants screened for enroll- ment into Phase I of the study over time, both cumulative and per month. doi:10.1371/journal.pone.0001954.g002 who were afraid of the testing procedures (n = 2). There were twenty-one women who were eligible following screening but never returned for enrolment into the HIV-negative cohort. Table 1. Demographic characteristics of the HIV-negative observational cohort. CAPRISA 002 Study The demographic characteristics of the 245 women enrolled in the HIV-negative cohort are presented in Table 1. The mean age of the overall cohort was 34.3 years (range 18–58). Approximately 42.0% of the cohort had 11 or 12 years of education or higher, 33.5% had between 8 and 10 years of education, and 24.5% had an education level less than 8 years. The numbers of steady and casual partners reported by the women are listed in Table 2. Steady partners were defined as someone seen ‘‘most of the time, often over a period of time’’ while a casual partner was someone the women saw only occasionally or even only once (but not a client in the case of the FSWs). All of the women in the cohort have engaged in peno-vaginal intercourse; 34.6% and 25.4% of the cohort have engaged in anal and oral sex respectively. The mean age at sexual debut was 17 years (range 12–26) and at the time of study enrolment the mean number of days since last sexual contact was 4.8 (range 1– 45). While 58.8% of participants indicated that a condom was used at last sexual encounter, 13.9% and 34.3% of the women indicated that they were never able to insist on a condom being used with casual and steady partners respectively (Table 3). Additionally, a high proportion of the women (75.7%) provided economic support to adults (mean 0.9, range 0–7) or children (mean 2.8, range 0–14). Among the women who self-identified as FSWs, the median length of sex work was 3 years (range 1 month–31 years) and the median age at which the women began sex work was 26 years Race/ethnicity Black White Indian Education . Grade 11 Grade 8–10 , Grade 8 Number (N = 245) Percent Number (N = 244) Percent 98.4 1.2 0.4 42.0 33.5 24.5 Marital status Single, no steady partner Married Divorced Widowed Stable partner Multiple partners Separated 10 16 3 3 75 135 2 241 3 1 103 82 60 Steady 158 62 7 8 1 0 5 241 4.1 6.5 1.2 1.2 30.6 55.1 0.8 Percent 0 0 13.4 13.0 7.1 0.8 65.5 100 Partners in the previous 3 months # of partners reported Number Percent 0 1 2–3 4–6 7–12 .12 Total Too many to remember 2.9 65.6 25.7 3.3 0.4 0 2.1 100 Casual Number 4 14 166 33 3 0 19 239 Percent 1.7 5.9 69.5 13.8 1.3 0 7.9 100 Lifetime partners Steady Number Percent 2 9 42 80 20 7 82 242 0.8 3.7 17.4 33.1 8.3 2.9 33.9 100 Casual Number 0 0 32 31 17 2 156 238 (Note: where women refused to answer certain questions, the category total is less than the cohort total of 245.) doi:10.1371/journal.pone.0001954.t001 Table 2. Sexual partner history for the 245 women in the HIV-negative observational cohort. (Note: where women refused to answer certain questions, the category total is less than the cohort total of 245.) doi:10.1371/journal.pone.0001954.t002 PLoS ONE | www.plosone.org 4 April 2008 | Volume 3 | Issue 4 | e1954