A 28-year-old male farmer presented with fever for 5 days accompanied by petechiae, transferred from a rural clinic to Zhongshan Hospital of Shanghai Medical University in November 1993. Physical examination: temperature 38°C, BP 70/50 mmHg, marked facial and periorbital congestion, conjunctival edema and congestion, cord-like petechiae in the axilla, and significant renal region tenderness. The suspected diagnosis is epidemic hemorrhagic fever. Regarding the epidemiological concepts of this disease, which of the following is incorrect?