Or HaChaim on Exodus 16:20/אור החיים על שמות ט״ז:כ׳

1 וירם תולעים, it developed worms. The reason the Torah tells us about this prior to telling us that the left-over manna rotted is, that normally we find worms develop only in foods which are sweet whereas these do not rot and stink. Since we have been told that the normal taste of the manna was like that of wafers covered with honey, we would have expected it to develop worms but not to rot and to give off a foul odour. The Torah therefore tells us that not only did it develop worms but it also rotted and began to stink.

2Verse 24 proves that our interpretation is correct, seeing that in that verse the absence of the foul odour is mentioned prior to the absence of worms. By reporting matters in that sequence the Torah tells us that even the worms which would normally develop did not develop overnight.

3I have noted that the Mechilta does not explain things the way we do; perhaps the author felt that the development of the worms preceded the decomposure of the manna although usually decomposure is the cause of the worms emerging (even though sweet foods do not rot and create foul odours).

4I believe it is likely that the Mechilta felt that the message of the Torah in relating these details was that worms by definition are the product of something which has gone rotten. Onkelos translates the word ויבאש as רע ביש, "bad and evil." Evil may be defined as the result of ignoring or violating the commandments of the Torah; a person who commits sins will eventually sprout worms. The mystical dimension of the existence of worms, i.e. parasites, is the sin committed by Adam when he ate from the tree of knowledge. It was an interaction of the fruit and the air (which he had polluted by his sin) which produced the first worm. All this is alluded to in the strangely worded וירם תולעים, "it raised worms," the cause being ויבאש, that something sinful had been committed with the manna. The Torah confirms this by its long-winded report in verse 24 where the Israelites are described as conforming with the instructions Moses had given concerning the treatment of the manna which had fallen on Friday. The thrust of that whole verse is to inform us that since no violation of the law had been committed there was no rotting and no worms were generated. When we are told in various stories in the Talmud that the bodies of certain righteous people did not produce worms after death, this merely confirms the nature and origin of worms. Compare Baba Metzia 84 about the single worm that grew out of the ear of Rabbi Eleazar son of Rabbi Shimon bar Yochai. That single worm was attributed to a sin committed with that ear.