This paper advances a new criterion of a vulnerable population in research. According to this criterion, there are consent-based and fairness-based reasons for calling a group vulnerable. The criterion is then applied to the case of people with serious illnesses. It is argued that people with serious illnesses meet this criterion for reasons related to consent. Seriously ill people have a susceptibility to "enticing offers" that hold out the prospect of removing or alleviating illness, and this susceptibility reduces their ability to safeguard their own interests. This explains the inclusion of people with serious illnesses in the Belmont Report's list of populations needing special protections, and supports the claim that vulnerability is the rule, rather than the exception, in biomedical research.