Wikipedia
Persin
For an island in Bulgaria, see Persin Island
Persin is a fungicidal toxin present in the avocado. It is generally harmless to humans, but when consumed by domestic animals in large quantities it is dangerous.
Persin is an oil-soluble compound structurally similar to a fatty acid, and it leaches into the body of the fruit from the pits. Negative effects in humans seem to be primarily in allergic individuals.