Why are some fungi deadly?
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. Some harmless mushrooms have evolved to mimic poisonous species to gain the deterrent effect without the effort of synthesising toxins. Other 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 byproducts, so they just build up in their tissues.