What happens to the influenza virus every 20-40 years, and why? What are the recent example(s)?

The influenza virus is mutated every 20-40 years.

Explanation:

Large mutations occur every 20-40 years. This results in a new influenza virus to  which the human population has no protection. These large mutations are almost always followed by an influenza pandemic.

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the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.