Every occupant of a structure shall be responsible for the extermination of vermin infestations and/or rodents on the premises. Every occupant of a dwelling unit in a dwelling containing more than one dwelling unit shall be responsible for such extermination whenever the dwelling unit is the only one infested; notwithstanding, however, whenever infestation is caused by the failure of the owner to maintain a dwelling in a reasonable rodent-proof or reasonable vermin-proof condition.
(`89 Code, § 27.110)