There are three types of flu: A, B, and C. All of these viruses are dangerous to humans but types A and B cause epidemics, whereas type C results in sporadic cases. The antibodies that are produced when the patient contracts one type of flu build the patient’s immunity only against that particular type and do not protect the person from other types. Moreover, the flu virus is prone to constant change. The World Health Organization (WHO) observes the genetic mutations of the flu viruses every year and gives recommendations on the optimal antigen content of the vaccine accordingly.