Contact with a person with the flu is not necessarily contagious, it just creates a higher likelihood of infection. Contact with a person who has influenza is not necessarily contagious. On the one hand, influenza is a respiratory infectious disease that requires secretions from the infected person to enter the respiratory tract of the uninfected person through droplets to cause infection; on the other hand, it is related to the titer of influenza virus that the infected person carries, which is higher in order to cause infection, and it is also related to autoimmunity. Therefore, it is not inevitable that exposure will result in infection, but if protection is in place, the titer of the virus is low, and autoimmunity is strong, then infection may not occur. The likelihood of infection is simply greater than for people who have not been in contact with a person with influenza.