Natural immunity can be acquired from a source of infection such as other people or sometimes things they have used. SF is very infectious usually via droplets in the air. Specific immunity to SF can be naturally acquired. I nursed SF cases in 1948 in a fever hospital. Nurses took a skin test to find if they had acquired natural immunity , and most young nurses had naturally acquired immunity.
It stands to reason that when SF is a rare occurrence people don't get a chance to acquire natural immunity.
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