Illustration of the immunologic reaction of the respiratory mucosa to an infection by influenza viruses, which weaken the human immune system. Influenza is a very contagious acute infectious disease of the respiratory tract that can occure endemically, epidemically, and sometimes pandemically (worldwide). The several types of influenza viruses are primarily transmitted via droplet infection. The portals of entry are the mucosae of the upper respiratory tract (see upper picture: germs permeate the weakened mucosa). The body reacts with special antigen-antibody-mediated reactions, with the common symptoms: Fatigue, fever, cold. The invading germs are recognized by the immune system.