[nudephil (OP)]

A question from Rick:

Why do we acquire lifelong immunity against some pathogens but not others? What is the biological mechanism underlying immunological memory?

[evan_au]

Our immune system becomes less effective as we age, so pathogens that previously were kept under control may come back to bother us.

Some pathogens like the influenza virus have a way of rearranging their genetic material to produce novel strains to which humans have little or no immunity.
- This is very likely to happen when a host animal is simultaneously infected by two different strains of influenza; this can produce virus particles which are hybrids of the two older strains.
See: https://en.wikipedia.org/wiki/Reassortment

Some diseases like HIV attack the immune system directly, locking onto the CD-4 receptors and invading the cells which form part of the immune systems memory (consisting of Memory B & T cells).
See: https://en.wikipedia.org/wiki/Immunological_memory

Another important part of the immune system memory is selective forgetting. T-Cells reacting to many possible antigens are generated by permutations of the genetic code. In the Thymus, those which react to the body's own cells are weeded out, to reduce the chance of auto-immune diseases.
See: https://en.wikipedia.org/wiki/Thymus#Function