A global outbreak of influenza seems to occur every few decades, killing millions of people at a time. At least one million people died in 1968. 1918-1919 (flu Spanish) has been more than 1 people in the world. This is because the flu virus is very easy to change. When a certain type of influenza A virus changes, it will produce a new kind of virus, which makes it difficult or unable to resist human beings, which led to a large outbreak of influenza.
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