The existence of viruses was suspected by Pasteur but it was not until 1897 that active work was begun on these agents. At this time Loeffler and Foesche demonstrated the existence of the virus of foot and mouth disease. Early workers depended upon the filter for the isolation of viruses and an attempt was made to grow viruses in artificial tissue cultures. Diagnostic tests were dependent upon the complement-fixation test and animal protection and neutralization tests.