Most states have procedures to force landlords to take care of condiditons like this. Maryland is no exception:
"Md. REAL PROPERTY Code Ann.
§ 8-211. Repair of dangerous defects; rent escrow
(a) Purpose. -- The purpose of this section is to provide tenants with a mechanism for encouraging the repair of serious and dangerous defects which exist within or as part of any residential dwelling unit, or upon the property used in common of which the dwelling unit forms a part. The defects sought to be reached by this section are those which present a substantial and serious threat of danger to the life, health and safety of the occupants of the dwelling unit, and not those which merely impair the aesthetic value of the premises, or which are, in those locations governed by such codes, housing code violations of a nondangerous nature. ...
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(e) Serious and substantial defects and conditions. -- This section provides a remedy and imposes an obligation upon landlords to repair and eliminate conditions and defects which constitute, or if not promptly corrected will constitute, a fire hazard or a serious and substantial threat to the life, health or safety of occupants, including, but not limited to:
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(3) Infestation of rodents in two or more dwelling units;
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(g) Notice by tenant. -- In order to employ the remedies provided by this section, the tenant shall notify the landlord of the existence of the defects or conditions. Notice shall be given by (1) a written communication sent by certified mail listing the asserted conditions or defects, or (2) actual notice of the defects or conditions, or (3) a written violation, condemnation or other notice from an appropriate State, county, municipal or local government agency stating the asserted conditions or defects.
(h) Reasonable time for repair. -- The landlord has a reasonable time after receipt of notice in which to make the repairs or correct the conditions. The length of time deemed to be reasonable is a question of fact for the court, taking into account the severity of the defects or conditions and the danger which they present to the occupants. There is a rebuttable presumption that a period in excess of 30 days from receipt of notice is unreasonable.
(i) Refusal by landlord to make repairs or corrections; action of rent escrow. -- If the landlord refuses to make the repairs or correct the conditions, or if after a reasonable time the landlord has failed to do so, the tenant may bring an action of rent escrow to pay rent into court because of the asserted defects or conditions, or the tenant may refuse to pay rent and raise the existence of the asserted defects or conditions as an affirmative defense to an action for distress for rent or to any complaint proceeding brought by the landlord to recover rent or the possession of the leased premises.
(j) Relief -- In general. --
(1) Whether the issue of rent escrow is raised affirmatively or defensively, the tenant may request one or more of the forms of relief set forth in this section.
(2) In addition to any other relief sought, if within 90 days after the court finds that the conditions complained of by the tenant exist the landlord has not made the repairs or corrected the conditions complained of, the tenant may file a petition of injunction in the District Court requesting the court to order the landlord to make the repairs or correct the conditions.
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Bottom line: you give the landlord written notice as described. If the problem isn't fixed within 30 days, withhold rent.