Their endeavours were, however, not crowned with complete success; on which account their powers were increased, in the year 1504, by granting them the right of life and death over those who violated the regulations.
Bills of health were probably first introduced in the year 1527, during a fatal plague which visited Italy for five years (1525-30), and called forth redoubled caution.
The first lazarettos were established upon islands at some distance from the city, seemingly as early as the year 1485.

Here all strangers coming from places where the existence of plague was suspected were detained.

If it appeared in the city itself, the sick were despatched with their families to what was called the Old Lazaretto, were there furnished with provisions and medicines, and when they were cured, were detained, together with all those who had had intercourse with them, still forty days longer in the New Lazaretto, situated on another island.

All these regulations were every year improved, and their needful rigour was increased, so that from the year 1585 onwards, no appeal was allowed from the sentence of the Council of Health; and the other commercial nations gradually came to the support of the Venetians, by adopting corresponding regulations.

Bills of health, however, were not general until the year 1665.
The appointment of a forty days' detention, whence quarantines derive their name, was not dictated by caprice, but probably had a medical origin, which is derivable in part from the doctrine of critical days; for the fortieth day, according to the most ancient notions, has been always regarded as the last of ardent diseases, and the limit of separation between these and those which are chronic.