Influenza virus is a virus of the family Orthomyxoviridae , which is responsible for influenza disease. At different times it has been identified as the cause of death of millions of people. Between 1918 and 1919, influenza killed about 50 million people worldwide. This terrible epidemic was then called "Spanish Flu " . At various times since then, new influenza viruses have emerged that have been identified as the leading cause of death worldwide. Different strains of the same virus can be used by humans , birds , pigs, etc. as their hosts . The most devastating effect of the influenza virus is its ability to change its host. Because the new host species does not have the immune system against influenza, it quickly enters the body , infecting other members of the species.