One type of antigenic variation is due to many pathogens having "a wide variety of antigenic types" (they have different serotypes) and so the body can acquire type-specific immunity to a pathogen. This means that the immune system will have antibodies that only recognise one serotype of a pathogen, and any other serotype of that pathogen will be recognised as a unique organism by the immune system. Therefore the same organism can theoretically cause disease many times in the same individual.
Another method of antigenic variation is caused by point mutations in the genes of a pathogen that code of an antigenic feature. This can be things such as cell surface proteins or the structure of polysaccharide capsules. This way, over time, an organism that an individual once had immunity to will have changed enough to no longer be recognised by the antibodies that recognised it before. This is called antigenic drift .