A 5 week old chick that seems to be sick

[spirited-away]

I have just recently bought 8 5 week old chicks. 4 Rhode Island Reds and 4 Australorps. However, one of the Australorps seems to be smaller then the others, doesn't eat much, drink much, is lethargic and always seems to be sleeping. The chicks have already been vaccinated for Marek's. I was afraid the other chicks might have been bullying it, so I have separated it from the rest. It still doesn't do much except for sleep. Would it be a disease, or would it be just a runt?

 

[rooster-red]

It might just be a runt, but I'd keep a close watch on it for any signs of sickness.