Many fungi have evolved toxins to stop animals eating them. These often advertise their toxicity with bright colours or a specific smell – the fly agaric toadstool is red with white spots, for example. Harmless mushrooms have evolved to mimic poisonous species to gain the deterrent effect without the effort of synthesising toxins. Some species are probably poisonous purely as an accident of their metabolism. Fungi digest plant matter into various chemicals, but can’t necessarily get rid of all the by-products, so they just build up in their tissues. But the most dangerous toadstools belong to the genus Amanita, which includes the death cap. These produce amatoxins, which stop your cells from making the DNA-like substance, mRNA. This causes the cellular machinery to grind to a halt, resulting in liver failure. In severe cases it is fatal, unless you receive a liver transplant.
Answered by Luis Villazon
Image: Tony Wills