PLEASE HELP! I have a number of baby chicks that are sick.

I have a number of baby chicks. They have done wonderful for the first couple weeks. Then all of a sudden on became rather lethargic. 3 days late 3 more now another one. They fall all over the place and lay on there side. They will still eat and drink but can not get around very well and look like are dead half the time till I touch them. I have tried sugar water. They are eating chick started feed that is medicated. I even bought save a chick electrolyte packets for in there water. I have now purchased tetracylcine to try and mixed it with there water today. I don't want anymore to get sick. I am hoping to save the ones that are. I have 50 babies. 25 that are a few weeks old and 25 that are a few days old. I am worried and can not find any answers as to what would cause this. They have plenty of space and I have multipul brooding lights. They are always scattered everywhere sleeping when I go out. So they are not to cold. It seems like it spreading what ever it is. Please help!