When a statement about the occurrence of a medical condition is qualified by an expression of probability, such as the word possible, listeners interpret the probability of the condition as being higher the more severe the condition. This severity bias can have serious consequences for the well-being of patients. We argue that the bias is due to a misconception of the pragmatic function served by the expression of probability. The more severe the condition, the greater the chance that the listener construes the expression as a politeness marker rather than as an uncertainty marker. When this misconception does not occur, neither should the severity bias. An analysis of interpretations of probability expressions using a membership-function approach validates this account. We discuss the consequences of this bias for the communication of risk within and outside the medical domain.