Why does Miss Leefolt lie about Minny having disease to Mae Mobley?

The shared use of toilets and bathrooms by black and white people is a recurring theme in The Help. Throughout the novel, Hilly Holbrook launches a legal crusade against shared bathrooms between whites and blacks, attempting to have a law passed that would require white citizens to build separate, outdoor bathrooms for their black employees. Hilly convinces Elizabeth Leefolt (Mrs. Leefolt) to build a separate bathroom for her maid, Aibileen. Mae Mobley, who adores Aibileen, is being potty trained and tries to use Aibileen's new toilet, which infuriates Mrs. Leefolt. She scolds and spanks Mae Mobley, telling her that she cannot use Aibileen's toilet, because Aibileen has diseases.
This belief that black people carried certain diseases that could be spread through contact with things like toilet seats was a common false belief throughout the South during the 1960s. This false claim was used to justify segregation not only of restrooms but also of drinking fountains, public transportation, and restaurants. By claiming that black people carried certain germs and diseases, white people had a scapegoat for the obviously racist policies of segregation. Mrs. Leefolt passes these lies on to Mae Mobley not only as a way to explain segregation but also as a way to perpetuate it, leaving Mae Mobley to grow up believing that segregation is justified and even necessary.