Scruffy and sick Blue Tit

Got a really scruffy looking "Bue Tit" ??  It was siting in the bird feeder with it's eyes closed.  I managed to pick it up with ease and it's now resting in a box on a heat mat,  Our concern is that it may have a disease and we're wondering whether it can be released.  We know it will probably not make it but if it may have something it can pass to the other birds.  Any thoughts?