hi kirsty b
when i first started with chickens i lost them all every morning i went to leave them out i nearly always found one dead the vet said they had a resporaty disese and the cure would cost more than the birds were worth. Hopefully you do not have this .the birds could be seen to be panting for air. After this i always buy vaccinated chicks . As a tonic the vet said i should a a bruised clove of garlic to there drinking water
best of luck
nemo