unexplained

I posted a couple of weeks back about some of my flock being sick. I treated them with Corid and I did a round of dewormer for good measure. The sickest ones have mostly recovered and I hadn’t noticed any other sick chickens until the other day when I posted about my chicken with the pale comb...

Hoping someone can help me try and figure out what might have happened... In brief: 8 week domestic drake, found dead, no sign of illness or injury in good weight, last seen in apparently good health, energetic, eating well, normal everything, 2 siblings have suddenly started limping at same...