This "Spitting on Floor or Walls Prohibited by Order of State Board of Health" dates to the early 1900s. The signage campaign was started at the urging of the members of the Ladies’ Health Protective Association, a group from New York City’s East Side, and later, the Young Women’s Christian Association.
The campaign was started after the discovery of Mycobacterium tuberculosis in 1882 by the German bacteriologist Robert Koch. Tuberculosis killed 1 in 7 people in the U.S. and Europe during the late 1800s, making it the deadliest infectious disease at the time. By linking tuberculosis to a bacterium, Koch opened the door for public health campaigns that aimed to prevent its spread.
At first, the city put up these signs in street cars reminding people not to spit and encouraged residents to remind each other not spew their saliva on the streets. But in 1909, a new health commissioner decided to enforce the ordinance more forcefully. On random nights, he instructed health officers to arrest anyone they saw spitting on subway platforms. The Sanitary Squad, as the officers were called, would round up hundreds of alleged spitters. They were brought to court together and subjected to fines of up to $2. When that proved ineffective, the health department also began to hand out informational pamphlets about the purported dangers of spitting.
The sign pictured above was manufactured shortly after World War I.