Young Chicks Suddenly Can't Stand

[Taylormade Chic]

I have some young chicks that are all feathered out and getting ready to go outside. suddenly yesterday I found one unable to stand. He eats and drinks fine, but can't stand up. This morning I found another with the same problem. Could it be a deficiency in calcium or something that is causing this?

 

[boskelli1571]

Are they vaccinated for Mareks?

 

[Taylormade Chic]

No, I just hatched them last month and I don't have the vaccine for Mareks. Does that act like this? They look like the photos I found on poultrypedia.com I found that after posting here and it is caused by vit B2 deficiency. But it doesn't say if they can get over it with Vit B2 or how long if so.