Malbim Ayelet HaShachar Klal 319
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ובכל זה כשאומר "יטמא" בפעל הוא לרוב ראשון לטומאה, לא אב הטומאה. אם לא כשבאר טומאתו בפירוש כמו "יכבס בגדיו, יטמא שבעת ימים", או הודעה אחרת (מצורע ס' קפ)
And nevertheless, when it says יטמא (shall be unclean) as a verb, it is usually referring to first-grade uncleanness, not the father of uncleanness, unless it clarifies his uncleanness explicitly, like יכבס בגדיו, יטמא שבעת ימים (shall wash his clothes…he shall remain unclean seven days)⁶¹² or through some other proclamation.
אב הטומאה*, which means “the father of uncleanness” is the highest level of uncleanness besides the uncleanness of a corpse.ראשון לטומאה, which translates into “first-grade uncleanness”, is the uncleanness which comes from touching “the father of uncleanness”.*
footnotes: ⁶¹² Leviticus 15:22-24.