The flu virus skips the first two steps. The flu shot is a killed vaccine against three strains of influenza virus. The flu vaccine is made ... There is a viral change over time through mutation and reassortment. A person can never become naturally immune to the flu because is always changing. It becomes impossible for our bodies to make antibodies against the different viruses. When people have a lower immune system the flu virus can easily enter the body and attack the cell. The flu shot is the most effective way to prevent influenza and its complications. In virus classification influenza virus are RNA virus that make up to the three of five genera of the family orthomyxoviridae. The type A virus are the most infectious human pathogens among the three influenza types and cause the most severe disease. Type A flu viruses are capable of infecting people as well as animals. Wild aquatic birds are the natural host for a large variety of influenza A. This A virus can be subdivided into different serotypes based on the antibody response to these