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The flu, short for influenza, is a highly contagious respiratory illness caused by influenza viruses. Understanding its contagiousness is crucial for effective prevention and management. Generally, individuals infected with the flu virus can spread it to others even before they exhibit symptoms themselves. The contagious period typically begins one day before symptoms develop and can last up to seven days after becoming sick. In some cases, particularly among children and individuals with weakened immune systems, the contagious period may extend even longer. 

Practicing good hygiene, such as frequent handwashing and covering coughs and sneezes, can help limit the spread of the virus. Additionally, getting vaccinated annually is the most effective way to prevent both contracting and spreading the flu. It's essential to stay informed about flu outbreaks in your area and take necessary precautions to protect yourself and others from this contagious respiratory illness.

More information check this tutorial: how long is the flu contagious