Are caused by the following: Water, Air, and Humans. If you would like a better answer please elaborate on your question. :)
Into the human digestive system, mucus is used as a lubricant for materials that must passover membranes e.g, food passing down the esophagus. A layer of mucus around the inner walls of stomach is vital to protect the cell things of that organ from the highly acidic environment with in it.
Sexual production is a means of recreating more of a species. Once there is a gene change whether for the good or bad, and solely if the organism survives to reproduce its gene, then it will be passed on to its children and going further on until there is a large amount of a species with that specific gene. This species evolved. Evolution occurs because of gene changes.