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It’s the stuff of sci-fi nightmares: a fungus that infects people and turns them into grotesque, cannibalistic zombies. Fortunately, despite its popularity as a video game series and now TV show, The Last of Us isn’t based on a true story and there’s no need to fear fungi and infection plunging the world into a bleak apocalypse.
But even if a cordyceps-caused dystopia is a long-shot, some questions remain: can fungi cause infectious diseases? What causes fungal infections in the first place? And what even are fungi, anyway? With Deakin University’s Emeritus Professor Leigh Ackland to guide us, let’s dig up the dirt on fungi.
Although fungi might seem superficially similar to plants, they’re actually very different and are the product of an entirely separate evolutionary pathway.
‘In terms of their taxonomic classification with all living things, they are in a kingdom of their own, distinct from plants, animals and bacteria,’ says Ackland. ‘Unlike plants, fungi are not photosynthetic but obtain their food through the absorption of nutrients produced by other organisms. They often live on decomposing organic matter.’
The most commonly known fungi come in the form of mushrooms (the classic toadstool image of the fly agaric is perhaps the most memorable) but the category is much broader, including yeast, mould and mildew.
Since culinary mushrooms are common (and, subjectively, delicious), we know that fungi aren’t all poisonous. If you’ve glanced at the news lately, though, you’ll know that certain types of fungi are poisonous.
As the triple murder trial of Erin Patterson suggests, some fungi can be highly poisonous and capable of causing death to humans if ingested. Death cap mushrooms (the fungi at the centre of the case) are responsible for some 90-95% of all mushroom-related poisoning deaths.
‘Some [fungi] produce toxins called mycotoxins,’ Ackland says. ‘Mycotoxins consist of a range of different toxic substances that are stable and are not readily broken down. Mycotoxins are produced by fungi as a way of inhibiting the growth of competitive species in their environment and to deter animals from feeding on them, as the fungi are living in environments where there is a lot of competition for resources like nutrients.’
Most mushroom-related deaths are caused by a type of mycotoxin called amatoxin (present in those death cap mushrooms, among others), which can present with symptoms like liver failure, encephalopathy (disease of the brain) and pancreatitis.
In addition to producing poisonous mycotoxins, fungi can also cause infectious disease.
Fortunately, though, we’re naturally resistant to fungal infection, making it relatively rare.
‘Not many of the total fungi that exist are harmful to humans,’ says Ackland. ‘Of the millions of species of fungi, only a tiny proportion (1 in 10,000) cause disease in humans. The reason why humans can resist fungal infections is partly due to the warm temperature of their body that may be unsuitable for fungal growth, the presence of the immune system and the difficulty fungi have in getting inside the body.’
That said, fungi still have the ability to cause infectious diseases in humans – and this can present some problems.
How do fungi cause infection and disease? There are a few ways, including ingestion, skin contact and breathing. Even still, there are four boxes that need to be ticked for fungi to infect humans:
Thankfully, the causes of fungal infections and disease are generally well-understood, and the infections are often easily treatable. Examples on this side include tinea and thrush, though there are some more serious examples of what fungal infections are capable of.
‘Cryptococcal meningitis is a serious fungal infection that affects the brain and spinal cord,’ says Ackland. ‘It is found in individuals with weakened immune systems. It causes headaches, confusion and stiffness in the neck that develop over weeks and can be fatal if not treated.’
Not all diseases caused by fungal infections are particularly harmful, but some types of fungal infections are contagious. ‘Particularly some types of fungal skin infections including tinea that prefers moist, warm conditions,’ says Ackland. ‘Tinea can spread through direct skin-to-skin contact or indirectly through shared items like towels, clothing or even floors in communal areas.’
If you’ve watched The Last of Us, you might have wondered if fungi could evolve to infect humans. While there’s no suggestion that it would cause a video game-style catastrophe (thankfully), Ackland says that fungi can and do evolve or mutate regularly.
‘Yes, fungi can evolve to infect humans,’ Ackland says. ‘They are in many ways similar to other microorganisms including bacteria and viruses that can mutate or change their DNA to help them adapt to their environment. Fungi generally mutate to a lesser degree than viruses or bacteria, but studies with aspergillus show that fungi can be hypermutators in terms of adapting to their hosts or resistance to fungal treatment.’
Some research has suggested that this hypermutation could be a problem not only by causing fungal infections and diseases in humans, but in agriculture, too.
Treating infectious diseases caused by fungi typically requires a specialised treatment of antifungal medication. These use specific actions to combat the fungal infection.
‘Some fungal treatments work by inhibiting or breaking fungal cell walls or by interfering with the growth of fungi,’ says Ackland.
Antibiotics are designed to fight bacterial infection – something they do very effectively. However, antibiotics do not work against fungi.
‘They are not effective against fungi which are a different type of organism from bacteria,’ Ackland says. ‘Fungal infections can be treated through other drugs that work through mechanisms from different antibiotics.’
While antifungal medications can be effective, it’s fair to say that most of us would prefer to prevent fungi-caused infectious disease in the first place. Ackland says that a person’s immune status can determine whether they’re likely to be infected in the first place but, regardless, there are a few easy ways to stave off fungal infections.
‘Fungal infections can be prevented by following the same procedures as those used to prevent bacterial infections, namely handwashing, avoidance of contaminated surfaces by wearing shoes in communal showers,’ Ackland says. ‘Also disinfecting surfaces and cleaning shared equipment.’
Don’t let all the talk of fungi causing infectious disease and zombie apocalypses put you off – fungi are our friends.
In terms of how fungi help humans, food is an obvious answer. Edible fungi include mushrooms (chanterelle, enoki, portobello and shiitake are a few examples) and the highly prized truffle.
Fungi like aspergillus can be used in cheesemaking and the production of foods like soy sauce and miso. Yeasts contribute to food and drink, too, with strains like saccharomyces cerevisiae (or brewer’s yeast) used for fermenting alcoholic drinks.
Perhaps the most impressive use of a fungus in helping humans is penicillium, the fungus that gives us the antibiotic penicillin. ‘The antibiotic effect of this fungus was discovered in 1928 by Alexander Fleming, and subsequently it was developed for mass production during World War II for the treatment of infections, saving millions of lives,’ says Ackland.
While it’s true that some fungi have the potential to cause harm or infectious diseases, there’s currently no need to worry about a Last of Us-style cordyceps apocalypse.
In general, remember that fungal infections are almost always treatable, rarely life-threatening and usually easy to prevent. Instead of fearing fungi, let’s celebrate this remarkable form of life!
Emeritus Professor,
Faculty of Science Engineering & Built Environment,
Deakin University
