The investigators are planning a study of independent cases and controls with 1 control per case. Prior data indicate that the transfer rate among controls is < 1%. If the true transfer rate for experimental subjects is 25%, the investigators will need to study 38 experimental subjects and 38 control subjects to be able to reject the null hypothesis that the transfer rates for experimental and control subjects are equal with probability (power) 0.8. The Type I error probability associated with this test of this null hypothesis is 0.05. We will use a continuity-corrected chi-squared statistic or Fisher's exact test to evaluate this null hypothesis.