Why does a person get sick with some diseases again?

1) After some diseases, antibodies are formed in the human body, immunity is formed. Antibodies against some diseases are destroyed over time, so a person can get sick again.
2) The causative agent mutates, old antibodies do not work against it (flu is an example).

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.