Our survey of 133 on-duty medical ward nurses showed that, during peak influenza season, 30 (23%) developed an influenza-like illness, despite wearing a mask at work. Suboptimal adherence to standard precautions, such as the use of a face shield (odds ratio, 3.56) during high-risk procedures, and failure to receive influenza vaccination (odds ratio, 4.82) were independent risk factors, adjusted for household contacts.