With the 19th-century discovery that diseases like tuberculosis and bubonic plague were spread by bacterial microorganisms, Victorians developed an obsession for cleanliness. Declaring war on an army of "invisible enemies," they fought an uphill battle against disease with such impotent cleaning agents as soda crystals, which, while serving a variety of purposes, still lacked disinfectant properties. The most effective weapon at a homemaker's disposal was the recently invented, human-powered, carpet sweeper. In addition to their biological foes, Victorians had to contend with a world covered in soot. Indoor gas lighting created a layer of dust that added an extra week to the average spring cleaning; outside, ubiquitous coal-burning fireplaces belched up gritty smoke that coated the landscape, entering the house through open windows or tracking in on the bottoms of shoes.