A 30-year-old male fisherman is admitted on September 10 with high fever, myalgia for 3 days, and hemoptysis for 1 day. Examination: temperature 39.2°C, BP 100/60 mmHg, mild irritability, conjunctival congestion, rales on lung auscultation, liver 2 cm below the costal margin. Laboratory tests: WBC 18.5×10<sup>9</sup>/L, serum agglutination-lysis test 1:400(+). Which drug is the first choice for pathogen treatment?