Assuming you have documentation showing that your rental was not safe for habitation you have a factual basis to fight the collection action against you. It would have been a better tactic to have had the local health department come in and cite the unit. If so, then you would have been in a better position to fight the action.Note, under the laws of all states in this country the landlord is obligated to mitigate his or her damages as to your lease. If the landlord has rented out the unit again that you occupied, you can claim as an offset the rent paid by the new tenant for the duration of the lease's time you had to reduce any amount that you might owe. Under the law, the landlord does not get double rent.