To touch the unclean made one unclean. As John J. Pilch noted in his book, The Cultural World of Jesus (pp. 35), ancient Jews concerned themselves with the notion of pollution, not of infection. They made little distinction between a cultural and a natural source of the pollution, as long as they kept the source of the pollution at a distance.
“In a culture that had a firm mistrust of change, they had an obligation to reject anything that changed what they believed was a God-given lifestyle. When Jesus touched the diseased man, he ‘took’ the disease upon himself. He changed the status of the ‘unclean’ to ‘clean’ and became polluted. He made himself rejected, so the leper could become accepted. And he did it willingly. In this sense, Mark foreshadowed Jesus' ultimate act of accepting pollution from nature and society. He took death upon himself, so all might have eternal life with God.”
From a reflection on the Gospel by Larry Broding