The causes I don't exactly know, but some doctors put everything down to the fevers.

Among the people, however, the one thought is poison, poison with all its legends, poison which kills like lightning and leaves no trace, the famous recipe bequeathed from age to age, through the emperors and the popes, down to these present times of middle-class democracy." As he spoke he ended by smiling, for he was inclined to be somewhat sceptical on the point, despite the covert terror with which he was inspired by racial and educational causes.

However, he quoted instances.
The Roman matrons had rid themselves of their husbands and lovers by employing the venom of red toads.

Locusta, in a more practical spirit, sought poison in plants, one of which, probably aconite, she was wont to boil.

Then, long afterwards, came the age of the Borgias, and subsequently, at Naples, La Toffana sold a famous water, doubtless some preparation of arsenic, in phials decorated with a representation of St.
Nicholas of Bari.