Does infection with Flu give me immunity? 

If you get the flu then you are only building antibodies to that particular strain of flu that caused the infection that made you unwell. This natural immunity is limited to that one particular strain. The virus changes over time, and new strains appear so protection from past infection or previous vaccination is unreliable. Until vaccines are developed that provide broader protection, the most effective way of increasing your protection is to be vaccinated every year. This flu jab will contain the most prevalent strains of flu and your body will create antibodies to the strains contained within the flu jab you have.