I read with interest the story of the Underground Quilts, and for what it's worth, here's what my mother and grandmother told me. We are from the bayous of Louisiana, and my grandmother would tell me that whenever there was someone with a contagious illness in the family, they would hang a quilt with a yellow symbol on the wall or the hedge. That was the universal sign of contagion and it meant "stay away". Conversely, if they were receiving visitors, a quilt with the pineapple design would be draped on the porch railing, the pineapple being the universal symbol of hospitality.
I agree with the poster who chooses to believe these stories, because a lot of family lore, myths, and stories are passed down by oral tradition. Slaves, and many poor whites, were not taught to read or to write in those times.