'fraid fitzcoraldo is right. Bracken "...causes cancer in sheep and cattle, and harbours a sheep tick that transmits a lyme disease to humans. It is invasive and endemic in upland regions, and has spread because its only enemy, cattle that trample on young shoots, are no longer kept on hillsides. It's estimated that 1,160 sq mi of countryside in England and Wales is now covered by bracken."*

Seem to remember that it's at its most carcinogenic when the fronds are just opening.

* Hutchinson Encyclopedia