Since each disease has a distinctive natural course of its own, the physician must make himself so familiar with it that he can predict the sequence of events and know whether and precisely when to intervene with treatment that will help nature to do its work.
Q) I was wondering why the underlined "and" is put in the sentence when it looks like without "and" the sentence sounds okay.
"and" is needed. The meaning is: know whether to intervene and know precisely when to intervene
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"and" is needed. The meaning is:
know whether to intervene
and
know precisely when to intervene