Each fall, the Rev. Rob Newells urges the congregation at Imani Community Church in Oakland, Calif., to get a flu shot. He builds bridges every day between the country's most vulnerable, marginalized communities and the medical system, defusing suspicion about HIV prevention treatments and educating people about medical research. He prods health-care leaders to think harder about their messengers: Don't send a white doctor to tell black people what they "need" to do for their own good.