The present invention provides methods for identifying a class of peptides referred to as replikins and methods of using replikins to stimulate the immune system of a subject. The method of identifying replikin peptides is based on identifying amino acid sequences comprising 7 to about 50 amino acids that contain (1) at least one lysine residue located six to ten residues from a second lysine residue; (2) at least one histidine residue; and (3) at least 6% lysine residues.