The classical theory that plague was caused by breathing in corrupt air was challenged by the Black Death, which was seen to be contagious. Civic authorities responded more quickly than the medical profession and were increasingly involved in attempts to prevent the spread of plague. Particularly influential was Milan, where a permanent Health Office, responsible for plague control, was established before the mid fifteenth century. Venice maintained a Health Office from 1490. At first its concerns were civic, but later it took responsibility for the whole Venetian mainland and for the work of provincial Offices. Plague control came to be conceived in broad territorial terms, and by 1600 the Health Offices of the Northern Italian states were in close cooperation to control disease. The Health Offices banned infected areas, preventing the movement of persons and merchandise by 'cordons sanitaires' and health passes. From the fifteenth century lazarettos were established, at first as hospitals for the plague sick and later as centres for quarantine and disinfection. Although hampered by administrative problems, the plague measures corresponded remarkably to the needs of the situation. The knowledge of plague gained inductively by the Health Offices stood in uneasy relation to theological and classical theories about epidemics. In the mid-sixteenth century Fracastoro and other physicians sought a synthesis. Writers on the epidemic in Venice and Padua in 1555-6 believed that plague might arise initially by corruption of the air and then spread elsewhere by contagion alone. In the plague of 1576, however it was doubted whether plague in which contagion alone was operative could be called 'true plague'. The resulting disputes revealed the continued vigour of classical theory in the late sixteenth century. The survival of theological views of plague was promoted at the same time by the gathering pace of the Counter Reformation.