That's not really enough info for us to be much help. If you look at the sticky at the begining of the emergencies section you will find a list of questions. Copy and paste those into a post (ask the mods to move this to emergencies) and answer as many of the questions as you can.
At 5-weeks old my first thought is cocci, but can't say for sure without a better idea of their diet, living conditions, etc.