Some pathogens have many different strains.
Influenza (common flue) is an example of a pathogen with multiple strains.
The different strains have different antigens- this is known as antigenic variability.
Memory cells will recognise antigens they have seen before and tackle a pathogen before symptoms arise- this is why you can only get chicken pox once.
However, if the antigen is different, the memory cell will not recognise it and be able to destory it.
This means that it is down to the slower and less effective primary response to kill the pathogen, allowing time for the pathogen to harm the body and cause symptoms- this is why you can get influenza multiple times.
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