In the courtly love tradition (for more on which, see "In a Nutshell"), it was common for poets to speak of love in terms of physical pain, wounding, or illness. They get all dramatic about it. We see this in spades in "The Knight's Tale."
Need an example? Sure you do. When Palamon first sees Emily, he cries out 'A!' as if he's been hurt, and later tells Arcite, "I was hurt right now thurgh-out myn ye / Into myn herte, that wol my bane be" (237-238). The idea here is that the sight of Emily has entered Palamon's eye, and made its way to his heart like a poison. Arcite, too, receives the love "wound," so "that, if Palamon was wounded sore, / Arcite is hurt as moche as he, or moore" (257-258). Later, Arcite blames his suffering on Love, who "hath his firy dart so brennyngly / Ystiked thurgh my trewe careful herte" (706 – 707). So love is a wound, inflicted on the heart through the eyes or through the love-darts of Cupid.
And after falling in love, the guys act like they're physically ill. When Arcite is banished from Athens and can't see Emily, he grows pale and he's unable to eat, sleep, or drink. The only "cure" for this sickness is the beloved, who is described as the antidote to a poison.