According to Centre for Disease Control and Prevention (CDC), the symptoms of swine flu were similar to those of influenza and of influenza like illness. It includes fever, sore throat, body aches, headaches, chills and fatigue.

The 2009 outbreak has shown an increased percentage of patients reporting diarrhea and vomiting. In United States, CDC advised physicians to consider swine influenza infection in the differential diagnosis of patients with acute febrile respiratory illness.

The presumed pathophysiology indicates that influenza viruses bind through hemagglutin on to sialic acid sugars on the surfaces of epithelial cells, which typically affect the nose, throat and lungs of humans. The viruses are 80-120 nm in diameter.