Influenza or the flu is a virus that spreads with ease. Viruses are complex and their transmission can seem random; people contract the virus despite not knowingly coming into contact with a sick person. Understanding how the flu spreads and when it is contagious enables everyone to better avoid catching it.


1. When Contagiousness Starts

Influenza is usually the most contagious in the first three or four days after the illness begins. However, the beginning of the illness does not necessarily coincide with noticeable symptoms. Most adults are capable of spreading the flu a full 24 hours before symptoms develop. Symptoms usually develop one to four days after the influenza virus enters the body.

sick man coughing bed Moyo Studio / Getty Images


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