A bacterial infection may develop following infection with viral influenza. Signs of a bacterial infection include:

• Yellow, green, rust-colored, or bloody mucus that is coughed up from the lungs, especially while other symptoms are getting worse. Mucus coughed up from the lungs is a more serious symptom than mucus that has drained down the back of the throat (postnasal drip).
• Ear pain that lasts more than 24 hours or severe ear pain that lasts longer than 1 hour.
• A sore throat that lasts longer than 2 to 3 days despite home treatment and does not "act" like a cold.
• Sinus pain that persists despite 2 to 4 days of home treatment, especially if nasal drainage is colored rather than clear, and fever is also present.
• Nasal drainage that changes from clear to colored after 5 to 7 days of flu, while other symptoms (such as sinus pain or fever) are getting worse.
• A cough that lingers more than 7 to 10 days after other symptoms have cleared, especially if it is bringing up mucus (productive).

These infections may sometimes need treatment with antibiotics.