Harmful fungi can disrupt biological function of other organism causing adverse reactions. Examples include Claviceps purpurea
Claviceps purpurea can cause ergotism (St. Anthony's fire) to humans that consume rye infected with the fungus. Ergotism is attributed to the alkaloid that it produces.
Meanwhile, fungus like Ophiocordyceps unilateralis (Zombie fungus) can infect infects ants of the Camponotini tribe thereby altering its behavioural pattern.