The Medusoid Mycelium is a deadly mushroom that grows in the Gorgonian Grotto.
Medusoid Mycelium plays a critical role in the final three books: The Grim Grotto, The Penultimate Peril and The End. In which is revealed it was used by Gregor Anwhistle and quarantined in the Gorgonian Grotto.
Developed by Gregor Anwhistle, this deadly fungus has a unique growth cycle in which it waxes and wanes. The fungus will undergo a brief dormant period when the mycelium, which branches out like bunches of thread, is nearly invisible. Then the fungus will begin its flowering stage, quickly and silently sprouting from the ground to full size. When it is time for the next dormant cycle, the fungus will wither away into only its mycelium form. 
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. Although it is referred to as a mushroom, it could actually be called a toadstool because of its poisons which render it inedible.
According to the following poem written by a V.F.D. volunteer, if a single Medusoid Mycelium spore is eaten, or even inhaled, it is enough to kill a victim within an hour:
- A single spore has such grim power
- that you may die within the hour
The spores will plant themselves inside the victim's throat and begin to grow thickly and rapidly inside. The blossoming mycelium will constrict the throat, choking its victim.[source needed]
The poison of the Medusoid Mycelium is unique, in that it is not poison in the sense of a toxin, or that it is unable to be digested, but because of its ability to grow on a wide variety of surfaces (e.g. the glass of the diving helmet, the human throat) where it would be impossible for other plants or fungi to grow. In addition, it thrives best in tight enclosed spaces, making the inside of a human body very convenient. Without this strange ability, the mycelium might not be poisonous at all, assuming there it has no other poisonous properties.[source needed]
- Is dilution simple? But of course!
- Just one small dose of root of horse.
Consuming this antidote will kill the fungus, leaving no traces of it behind inside the victim's body. Other spices and substances with the same bitter, strong taste or intensity are also able to stop such an infection. Violet and Klaus discovered wasabi as a cure for the poison when their little sister Sunny requested it, and they gave her the spicy condiment, which effectively killed the fungus and strengthened her breathing.[source needed]
In the past, V.F.D. made it a priority to make sure that there were plenty of locations where antidotes could be found, effectively protecting the world. The organization once processed horseradish in the Opportune Odors Horseradish Factory on Lousy Lane.
In an effort to find another cure, Bertrand and Beatrice Baudelaire created a hybrid tree, cross breeding horseradish with apple trees. This hybrid tree grew on The Island, and its fruit can dilute the poison with a single bite.
Perhaps yet another remedy is bitter tea, with the V.F.D. proverb stating Tea should be as bitter as wormwood and as sharp as a two-edged sword., which most members of V.F.D. drink exclusively. 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.[source needed]
Role in the series
In The Grim Grotto, it had been safely quarantined in the Gorgonian Grotto, but a sample was taken from the grotto in Sunny's diving helmet. Fiona then gives the helmet to Olaf to save her brother.Olaf keeps the biological weapon in the underwater. If the helmet was opened, it could spread the deadly fungus to the whole word and infect everyone.
In The Penultimate Peril, Olaf threatens to release the fungus in the lobby of Hotel Denouement. It is kept hidden in the helmet inside a ball on the roof.
In The End, the Islanders became infected and it is ambiguous if they survive. The exception is Ishmael who is immune because he has been eating the apples regularly. Kit Snicket perishes as a result of the infection, since she refuses to eat the cure. This makes Kit Snicket the only known victim to have died of the fungus.
- The Mycelium serves as possibly the main antagonist (other than Count Olaf) in the final three novels in the series, due to Olaf's threats to use it as a biological weapon.
- As Snicket states in his narration, V.F.D. was most likely an ancient Greek idea, (given his description of the Cimmerians and as he mentions Medusa the Gorgon as being the namesake of the Gorgonian Grotto) this possibly means that Perseus was one of the first volunteers in V.F.D. and that he sealed the Mycelium (represented in the story by Medusa) into its cave and thus stopped further infection. So, the Gorgonian Grotto was named after Gorgon, and Medusoid was named after Medusa.
- In The Bad Beginning, Justice Strauss says that she had a case concerning a poisonous plant. This may be a reference to Medusoid Mycelium, however as mushrooms are fungus, not plants, this is not guranteed.