In an earlier chapter, Kassan said he had been healthy all his life. He lived in a warmer climate, however. So he got a bad cold twice--once after Igen kicked him out, he waited in the rain not knowing where to go; then a second time after smexy time with Igen when they became official lovers. Keep in mind that he may have been more susceptible to illness due to the drug that was in his system. He's been drugged twice in the series. All that was out of his system before he was slowly poisoned with bitchies' treats.
Even if all the drug was out of his system it would have still effected the balance of his body. Like a once healthy and strong horse can have ailments somewhere in life. Just saying ^^
Also even though building up resistance to poison is possible:
"It is important to note that mithridatism is not effective against all types of poison (immunity generally is only possible with biologically complex types which the immune system can respond to) and, depending on the toxin, the practice can lead to the lethal accumulation of a poison in the body. " (from Wikipedia)
So it also depends on the poison itself. Those poor birds only ate a piece and died on the spot. At least it took Kassan longer before it takes him down. So he's still strong to that point
She did not realize that Kassan was the one getting poisoned since the treats were for Igen. She assumed that the poison was less effectual because Igen had an immunity to them. Unfortunately, Kassan hid them from Igen and ate the treats because he was jealous. Thank god he started throwing them out the window or he would have surely died

Bitch was feeding it to kassan in small dosages so why didn't he just build up a resistance to it? Can't read the raws but something tells me his prognosis isn't good.
I had to go back to the raws bc I needed that last panel. Igen was like slash. Bitch now you fugly be grateful I let you live.