Whenever any building, or part thereof, in such cities, shall, because of age, infection with contagious disease, defects in drainage, plumbing or ventilation, the existence of a nuisance on the premises, or conditions tending to cause sickness among its occupants or among the occupants of other property in said city, or constituting a danger to the life or health of the occupants of other buildings in its vicinity, or because it prevents proper measures from being carried into effect for abating any nuisance injurious to health or sanitary evils in respect of such other buildings, be unfit for occupation or a menace to public health; and when such dangers and evils in, or caused by, said building cannot be removed by repairs, or in any other way, excepting by the destruction of the said building, or any portion of the same, the said Department of Public Health may order the same, or any part thereof, to be removed. The said order shall specify the building, or part thereof, to be removed, the reason or reasons therefor, and shall also specify a reasonable time within which said work or removal shall be commenced and prosecuted to completion.