Depiction of the immune reaction to infection with influenza virus, that weakens the human immune system. Influenza is a very infectious acute disease of the respiratory tract that occurs endemically, epidemically, and sometimes even pandemically (worldwide). It is caused by various types of influenza viruses that are transmitted via droplet infection. The portal of entry are the mucosa of the upper respiratory tract (left). The body reacts with specific antigen-antibody mediated reactions (right) with the well-known symptoms: lethargy, fever, cold, joint pain.