Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Worms 9.34): Fleas [pulices], as the Liber rerum says, prefer to jump rather than fly on a dark color. And these also are a scourge to man, and given as a punishment to the smallest and most wretched animals. For it attacks a man more at night than during the day, and when there is a severe torpor or deep sleep in his limbs, he cannot protect himself. When any one uses his hand to avenge himself, they suddenly flee and cannot be found. When it is thought that there is peace from them, they quickly return to attack, knowing it is not possible for a man to catch them, even when driven by fury. They pierce a man with a sharp bite, and draw blood by the power of their throat, but from the entire skin they can only take blood through the pores and from natural evaporation. They are more active in the moist air and the softened earth. They are said to be produced from dust heated and then corrupted by moisture. Against fleas the main remedy is the whole body rubbed with absinthe in the evening, as Ambrose says: Fleas will not touch you if you boil the herb of absinthe with oil and anoint your body. As the Experimentator says, fleas leave a mark on the flesh of the man on whom they alight. A flea was divided into two parts and lived. If it is cold around the month of June, the fleas will quickly die and there will not be an abundance of them for the rest of the summer. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book18.88): The flea is a little worme, and gréeveth men most, and is called Pulex, and hath that name of Puluis, pouder, for it is namely fed with pouder, as Isidore sayth, libro. 12. And is a little Worme of wonderfull lightnesse, and scapeth & voydeth perill with leaping, and not with running, and waxeth slowe, and fayleth in colde time, & in Summer time it wereth nimble & swift. And though it bée not accounted among beasts that be gendred, and knowen among beastes by medling of male & female, yet he multiplyeth his owne kinde by bréeding of Néetes: For they bréede certeine neets in themselves, and of that commixion or comming of Néets, many Fleas do come of one Flea. And the Flea is bred white, and chaungeth as it were sodeinelye into blacke coulour, and desireth bloud, and biteth and pearceth therefore, and stingeth the flesh that hee sitteth on, and sucketh the thinnest parte of humours that bée betwéene the skinne and the flesh, and maketh in that parte of the bodye, in the which he sucketh, a bloudie token, and doth let them that wold sléep with sharpe biting, and spareth not kings, but a little Flea gréeveth them, if he touch theyr flesh. And to Fleaes Warmewood is venim, and so be leaves of the wilde Figge trée, as Constantine sayeth. And Coloquintida, a wéede that is lyke to a wilde Nep, helpeth against Fleas, if it be stamped and medled with water, and sprong in the place there as many Fleas be: and so doth Wormewoode leaves, for as it is said, they die by smell & savour of wormwood: and by swiftnesse of leaping, they be the worse to take, & they bite full sore against raine. - [Batman]