Sentence Correction (SC) | Re: The English physician Edward Jenner found that if experimental subject

I have understood that 'E' is the correct answer.
However, I am not sure of the below comment in OG-
Th participle causing suggests that infecting
the subjects with cowpox caused a mild
illness only in some of the cases

Can someone please elaborate on the above remark?
causing only a mild illness, they would become immune to' - is it still wrong?

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