In the makkot, there seems to be inconsistency in terms of the number-title of the plagues. One of the words is in the singular (tz'farde'a, frog) while the one after it is in the plural, kinim, lice.
Rashi gives two answers as to why the 2nd plague is in the singular and references the 3rd:
Heb. וַתַּעַל הַצְפַרְדֵעַ, literally, and the frog came up. It was one frog, and they [the Egyptians] hit it, and it split into many swarms of frogs. This is its midrashic interpretation (Tanchuma, Va’era 14); for its simple meaning, it can be said that the swarming of the frogs is referred to as singular, and likewise, “and the lice were (וַתְּהִי הַכִּנָם) ” (verse 13), the swarming, pedoiliyere in Old French, swarming of lice, and also וַתַּעַל הַצְּפַרְדֵּע, grenoylede in Old French, swarming of frogs.
But his explanation doesn't seem to fit -- the singular of lice in English is louse, in Hebrew, kinah according to Milon Morfix and Ken according to Even Shoshan (as in Even Shoshan's understanding of Yeshayahu 51:6, which is substantiated by the Malbim Beur Hamilot on Sefaria, " נוכל לאמר כי מלת כן הוא הנפרד מן כנים, כמו קן מן קנים, ככנה ימותו"), not kinam.
Kinam, it seems, is a specific word for "swarm" -- Rashi says it is a collective word treated as the singular in pasuk 13.
So if the simple singular is used for "frog infestation", why not use the simple singular for "louse infestation"? And if there is a technical term for a swarm of lice, why not use that instead of the basic plural?