Photo Credit: Chris 73/Wikimedia Commons
1348 is where the real nastiness comes in, when Poveglia was used to quarantine people afflicted by the Bubonic Plague — which killed one out of three Europeans. Anyone who showed even the slightest symptom (plus those who had already died) was taken to the island and left there, whether or not they actually had the plague. The same thing was done in 1630 when the Black Death hit. Later, the stories and legends surrounding the island were so scary and effective that Napoleon's military campaign even left gunpowder and weapon supplies there for protection.
And it gets worse. In the late 1800s, all of the local mentally ill were taken to an asylum built on the island (which was really just another form of exile). There are even rumors that a doctor used to experiment on patients before going crazy himself and jumping off the bell tower. People on the mainland report hearing the chimes echoing from the island, even though the tower was torn down long ago. The building was turned into a geriatric center in the mid-20th century, only to be closed in 1975, and since, the island has been left empty — future attempts to restore the old asylum were ceased without explanation and people are currently not allowed to visit Poveglia.