On November 6, a patient had the onset of an illness characterized by fever, chills, headache, cough, and chest pain. The illness lasted 1 week. On December 5, she had another illness very similar to the first, which lasted 6 days. She had no influenza immunization during this period. Her hemagglutination inhibition antibody titers to nH1N1 influenza virus were as follows:November 6: 10 November 30: 10 December 20: 160There was no laboratory error. Which of the following is the best conclusion from these data?

The symptoms described for her illnesses on both November 6 and December 5 are consistent with influenza. However, the November 6 illness (a) was not caused by nH1N1 influenza virus; the low titers (10 on November 6 and 10 on November 30) most likely represent cross-reacting antibodies from the agent that caused her first illness. The greater than fourfold rise in titer from 10 on November 30 (baseline) to 160 on December 20 reflects a definitive diagnostic rise in antibody against nH1N1 influenza virus. Thus, her December 5 illness (b) was influenza. There is enough serologic evidence to make the diagnosis (choice e is incorrect).