you could take it to the vet for a check-up and treatment of any illnesses if you plan on keeping it or you could take it out to a field and release it there. of course, its chances or survival are better if it becomes a pampered pet but it does not decrease much whether you release it in a field or leave it where it is. being released in a field it will need to find another colony and be accepted something that may or may not happen. being left where it is it may or may not be trapped or poisoned. if you chose to make it a pet, there will be considerable work ahead of you getting it to trust you. you will also have to take considerable more time then you would an already domestic rat to be tamed. however, as far as if this can be done or not, it most certainly can. pet rats, not too long ago, would be caught on th street in the morning and sold as pets that evening. if there is any disease, the vet, with blood tests, should be able to pick up on any. it would also be a good idea, if you decide to keep it, to treat it for mites and such right off the bat. living out in wild, its sure to have some parasites.