Abbreviation of 流行性感冒 (りゅうこうせいかんぼう). Used in medical and everyday contexts, though インフルエンザ is more common in modern speech.
See also: 流行性感冒
Influenza is going around, so please wash your hands thoroughly.
彼は流感で一週間学校を休んだ。
He missed a week of school because of the flu.
flu shot; influenza vaccination
Abbreviation of 流行性感冒 (りゅうこうせいかんぼう), literally 'epidemic cold'.