Does "down" have the meaning of "sick"? I think so, but I'm not sure.
Maybe "I'm down with flu" makes sense.
ex)Almost all the pet foods you will find in the supermarket is reconstituted animal by-products. Better known as low-grade waste from the chicken and beef industries, it is food that no one in their right mind would eat..... The foods are classified in the industy as 4D, which means dead, dying, diseased or down.....