The「illness of freshness」concept perhaps may be arrived at when considering that only freshly killed or butchered meat would still contain live parasitic worms…undeniably an added source of animal protein but only if chewed well prior to swallowing and digestion, whether desired or not, but it is certainly difficult to bite all the eggs, and just a few remaining intact can cause discomfort later. Nevertheless, to prevent an infestation of one's own intestinal tract, the best advice is either to certainly chew each bite thoroughly, or, to cook the flesh well before ingestion, that is,「eat your worms either fully cooked or fully chewed」as it is unlikely that all the worms can or will be removed whilst preparing a serving suggestion (however, see the warning below).
Normally, parasitic worms in muscle will migrate back into the intestines when a host dies, being the place where the worms have both the best chance of survival for the longest time within the carcass by feeding on feces, and the best chance of being ingested by a successor host, since scavengers normally begin eating a carcass from the softest point of entry into the dead body, the anal orifice, which is attached directly to the intestines (and feces). Therefore, if the worms are still in the meat, it is an unmistakable sign of freshness…maybe that is why formerly sashimi was sliced so thinly…yet, those crafty parasitic worms often roll up into little balls of「ringworm」indicative of one of evolution's finest hours. Warning: fully masticating live worms may not crush any worm eggs, so please enjoy your worms with sufficient care. Further warning: not all parasites are sufficiently large to be seen by the human eye, let alone sufficiently large to be chewed by human teeth.
「Physical condition caused by slippery meat with sheep」seems to be referring to parasites worming their way into one's penis meatus while tupping the animal's anus. Probably not a 'best practice.'

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: ringworm
Japanese EDRDG: ringworm
Unihan extended: ringworms