The Medusoid Mycelium is a deadly mushroom that grows in the Gorgonian Grotto and has a unique habit of waning and disappearing underground, then sprouting silently and very quickly back up to full size again (waxing).
The mushroom, like most mushrooms, has an umbrella-shaped pileus, and both the pileus and the stipe are dark gray, randomly speckled with black spots. It grows thickly in the throat, constricting the air-passage and choking the victim. However, this mushroom takes time to kill a victim, and so all is not lost if the antidote, horseradish, can be consumed to dilute the poison. V.F.D. made it its priority in the past to make sure that there were plenty of locations where antidotes could be found, effectively protecting the world.
Although it is referred to as a mushroom, it should actually be called a toadstool because of its poisons which render it inedible, though the poison may be removed by boiling as is done with many other 'mushrooms'. It had been safely quarantined in the Gorgonian Grotto, but a sample was taken from the grotto and kept by Count Olaf as a biological weapon in the underwater helmet worn by Sunny Baudelaire which, if opened, could spread the deadly fungus far and wide.
It is stated by Fiona that the fungus can actually be the source of some marvelous remedies.
Cure for the Infection
The cure for this infection is horseradish or a spice or substance with the same bitter, strong taste or intensity. Violet and Klaus discovered wasabi as a cure for the poison when their little sister Sunny told them, and they gave her the spicy condiment, which helped her breathing get stronger and cured her. V.F.D. once processed horseradish in the Opportune Odors Horseradish Factory on Lousy Lane. Bertrand Baudelaire and Beatrice Baudelaire also created a hybrid tree by cross breeding horseradish with an apple trees. This tree grew on The Island (which is sometimes called Olaf Land), and it can dilute the poison with a single bite.
A "member of V.F.D." wrote a poem about horseradish being the antidote for the Medusoid Mycelium:
"A single spore has such grim power That you may die within the hour. Is dilution simple? But of course! Just one small dose of root of horse."
Other Remedies
Horseradish and wasabi are not the only possible cures. The apple and horseradish hybrid fruits were also shown to counteract the poison. Another remedy seems to be bitter tea, which most members of V.F.D. exclusively drink. It has even been hinted that anyone mentioning having sugar in their tea was immediately considered a villain, although this could also be because of the battle between both sides to possess the sugar bowl.