Hospital-acquired influenza is associated with significant morbidity and mortality in hospitalized patients notably elderly patients. Furthermore, it is also associated with a large economic impact for the hospitals.

The transmission of influenza has been mostly reported in pediatric and long-stay units. The chains of transmission of influenza in acute-stay units have to be describe in order to prevent and control potential outbreaks. Furthermore, to know clinical symptoms seems to be important in order to identify potential sources of virus as soon as possible and to set up appropriate hygiene prevention measures. Moreover, the definition of the hospital-acquired influenza has to be harmonized for all over the studies, especially concerning the delay between the admission in the hospital and the symptoms onset.

The aim of this study is to describe the hospital-acquired influenza in a french university hospital of around 800 beds

Inclusion Criteria:

• Adult patients and health-care workers in a participating unit in the study,
• more than 18 years old,
• volunteers,
• who present an influenza-like illness defined by fever > 37,8°C or/and cough or sore throat.

Exclusion Criteria:

-Everyone who does not present the inclusion criteria.

Each patient or health-care worker who present ILI (influenza-like illness) at admission or during hospitalization. ILI is defined by fever > 37,8°C or/and cough or sore throat.