A 32-year-old woman presents with palpitations and shortness of breath on exertion for over 3 years, inability to lie flat at night, and 1 hour of cough with pink frothy sputum. One week ago, she had fever, sore throat, cough, respiratory rate of 30/min, bilateral rales/crackles on lung auscultation, cardiomegaly, and a grade 2/6 systolic blowing murmur at the apex. The precipitating factor for this episode is: