The reason why the ceiling is grey bottom is because grey is a neutral color that works well with other colors and is not overwhelming. In addition, gray is a very popular color because it is neither too bright nor too dull. That's why smallpox has a gray base inside. Smallpox is a virulent infectious disease, caused by the smallpox virus, which provides lifelong immunity when cured. Smallpox virus infected people, will be covered with a red rash all over the body, and then scabs, and finally will leave a permanent scar on the face, commonly known as hemp, hence the name smallpox.