A reservoir host is a lower animal which shares some disease or parasite with man. The infection or the parasite is equally at home in man and the reservoir, is biologically indistinguishable in either case, and may oscillate quite contentedly from one to the other as environmental circumstances open the way. Such is P. pestis in relation to rat and man, and E. granulosus in relation to sheep and man. At certain times and places man may erect barriers of absolute defense and restrict the disease to the reservoir host, as with vaccination and mosquito control in yellow fever, or by abstention from uncooked fish as in clonorchiasis. He may go a step further and attack both the disease and the reservoir host itself, as in bovine tuberculosis and brucellosis in this country. However, if this effort fails of complete success, and if cracks appear in his personal walls of defense, the disease sweeps again into the human host.