They are the result of slight changes in the genetic information that produces them.
Human evolution and natural selection reduce susceptibility of disease. A simple explanation would be saying that the life forms that are not immune to the disease pass away, then the ones that are immune or survive genetically give this trait to their offsprings, leaving only life forms that are immune. This does not occur so easily and is a long process, but it is a simple explanation.
Proteins are biological polymers constructed from amino acids joined together to form peptides. These peptide subunits may bond with other peptides to form more complex structures. Multiple types of chemical bonds hold proteins together and bind them to other molecules. Take a closer look at the chemical bonds responsible for protein structure.