Influenza type A viruses undergo two major kinds of changes. One is a series of mutations that occurs over time and causes a gradual evolution of the virus. This is called antigenic "drift." The other kind of change is an abrupt change in the hemagglutinin and/or the neuraminidase proteins. This is called antigenic "shift." In this case, a new subtype of the virus suddenly emerges. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift and therefore do not cause pandemics.
During the 2009 flu pandemic , many thousands of cases of ILI were reported in the media as suspected swine flu . Most were false alarms. A differential diagnosis of probable swine flu requires not only symptoms but also a high likelihood of swine flu due to the person's recent history. During the 2009 flu pandemic in the United States , the CDC advised physicians to "consider swine influenza infection in the differential diagnosis of patients with acute febrile respiratory illness who have either been in contact with persons with confirmed swine flu, or who were in one of the five . states that have reported swine flu cases or in Mexico during the 7 days preceding their illness onset."  A diagnosis of confirmed swine flu required laboratory testing of a respiratory sample (a simple nose and throat swab). 
1. Anxiety activates the stress response, and the changes the stress response brings about can mimic flu like symptoms.
Behaving in an apprehensive manner (worried, fretful, fearful) causes the body to produce the stress response . The stress response secretes stress hormones into the bloodstream where they travel to targeted spots in the body to bring about specific physiological, psychological, and emotional changes that enhance the body’s ability to deal with a threat—to either fight with or flee from it—which is the reason the stress response is often referred to as the fight or flight response .