Why did harmful medical treatments like bloodletting persist for centuries? We propose the following solution to this important cultural evolution question: Illness or injury often leaves people incapacitated for long periods. Caregiving - the provision of food, shelter etc. - is an important medical practice that enables ill or injured people and their dependents to survive these periods. However, caregiving is vulnerable to exploitation via illness deception whereby people feign illness in order to gain access to care. We propose that harmful treatments helped to solve the problem of illness deception because only individuals who stand to gain much from care will accept the treatment. To investigate the plausibility of this theory we develop a mathematical model using invasion analysis and replicator dynamics. We show that decreasing the benefit of caregiving via aversive and unpleasant medical treatments can counter-intuitively increase the range of conditions where caregiving is evolutionarily viable. Thus poisoning, cannibalism, emetics, bloodletting and other such treatments may be solutions to the problem of allocation of resources to people with unclear need.