cognitive dissonance

Cognitive dissonance is a psychological state that describes the uncomfortable feeling when a person begins to understand that something the person believes to be true is, in fact, not true. Similar to ambivalence, the term cognitive dissonance describes conflicting thoughts or beliefs (cognitions) that occur at the same time, or when engaged in behaviors that conflict with one's beliefs. In academic literature, the term refers to attempts to reduce the discomfort of conflicting thoughts by performing actions that are opposite to one's beliefs.
-- from Wikipedia



To sap the strength or energy of; enervate.
Having "the flu" is something of a generic complaint. Other common and less debilitating diseases are often mistaken for influenza. As a result, people tend to regard flu in casual terms, and to take it less seriously than they should---at least until they actually get the real thing!