I know you're upset right now..
but try and use this as a learning experience..
observe your chickens..
watch for any changes in behavior and droppings
the earlier you notice something, the better chance you have to save them
make sure they always have fresh feed, proper for their age
and fresh water.
a safe clean shelter and clean bedding
treat for mites
worm a few times a year, especially if they free range.
it's also a good idea to have a few basic meds on hand..
such as Tylan, or erythromycin
a cocci med
and first aid supplies, such as saline solution, also saline eye wash, Betadine, eye ointment, peroxide, neosporin, vaseline..