Influenza is an acute highly contagious disease, characterized by a sharp intoxication, mild catarrhal symptoms. Flu (like other acute respiratory diseases) is caused by different viruses that have numerous strains.
A period of acute clinical manifestations is the first period of influenza. This means that the patient in most cases can remember not only the date of the disease, but even he will specify the exact time(!). First flu-like symptoms appear suddenly after an incubation period. I mean that the period from the moment the virus enters the body of the future patient until the first signs of the disease extends from 12 to 48 hours (although the incubation period can sometimes last up to 5 days).