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Snapseed
the flu virus (influenza) constantly changes, or mutates, which is why it’s hard to have one vaccine to cure the flu. that vaccine will be ineffective to the new virus as it has adapted to counter the vaccine (such as changing the surface proteins of attachment)
miaxoxo66
The flu never just stays the same , there is always something different changing to it so there are always new changes needed to be made to prevent the flu.