Covid-19 proximity tracing apps can contribute to the fight against the pandemic. In a free society, however, their adoption is impossible to enforce by decree or material incentives. Their success therefore hinges on voluntary cooperation. We show that activating the app has considerable private benefits that many may underestimate – especially by offering information to guide their behavior towards vulnerable relatives and friends. Hence, activating the app need not primarily be seen as an act of social generosity. Alerting contacts through the app after having tested positive, however, presents a tradeoff of private costs against societal benefits. We argue that these private costs are likely to be negligible for most users, while the social benefits (saving lives) are potentially large.