We use a field experiment with 3,176 patients at 122 tuberculosis treatment clinics in India to test whether peer referrals increase screening and identification of patients with an infectious disease. Low-cost financial incentives considerably raise the probability that current patients refer prospective patients for screening and testing, resulting in the cost-effective identification of new tuberculosis cases. Incentivized referrals operate through two mechanisms: peers have private information about individuals in their social networks (beyond their immediate families) to target for outreach, and peers are more effective than traditional contact tracing by paid health workers in inducing these individuals to get tested.