The Last Of Us: Fungal infections are a growing threat, but don't worry, they won't turn us into zombies

The World Health Organization recently released a report stating that fungal infections present a growing threat to human life. For anyone who has been watching HBO’s TV seriesThe Last Of Us, this may be cause for alarm. But how big is the threat of fungal infections and how well-equipped are we to deal with them? We spoke to Dr Rebecca Drummond, a fungal immunologist based at the University of Birmingham, to find out.

Should we be concerned about fungal infections?

Yes, absolutely. I think we've been ignoring fungal infections for a very long time so it's really pleasing to see the World Health Organization making a point of the problem that we're having. Part of the reason is that fungal infections are not really natural infections of humans, we're quite resistant to them. Our immune systems are really good at fighting them. The problem comes when your immune system is damaged. This creates holes in our immune defences and then fungi can get in and cause problems.

What's happened over the last half century or so is that people who have damaged immune systems have increased. The AIDS pandemic is one example of what has caused this. You have lots of people who are now living with HIV who are very susceptible to getting fungal infections. Also, a lot of drugs we use to treat cancers have the side effect of damaging our immune system so we see fungal infections popping up in cancer patients as well. This makes them really problematic because we're dealing with a group of patients who are very difficult to treat and very clinically complex.

In The Last Of US, the global pandemic is caused by a mutated strain of the Cordyceps fungus that turns infected humans into mindless zombies. Could this ever happen in real life?

There's no evidence that Cordyceps infects humans. It mostly infects insects and there's a small number that seem to be able to exert some kind of mind control over their hosts. They enter the nervous system of the insect and control its movements. These fungi are very well adapted to their host, so you'll have one type of Cordyceps that will infect an ant, say, and another one that will affect a grasshopper, one that will infect a spider and so on.

Exactly how it works is not particularly well understood but what’s usually happening is that the fungus is trying to force the insects to go to a site where it can germinate and release more spores, and thus spread the infection.

What are the most common fungal infections in humans?

Probably the most common one that everyone will have heard of is thrush. This is caused by a yeast called Candida. Most of us actually have it in our guts. But certain things can cause damage to the immune system and allow Candida to start causing infection. A common one is taking antibiotics.

Thrush is really common, we think billions of people around the world get infections every year. It can also become very dangerous if the yeast gets into your bloodstream and invades different organs like the kidney and the liver.

Another common one that we see in the UK is Aspergillosis, a mould infection in the lung. This tends to affect people who have lung problems like cystic fibrosis or lung transplant patients. And then probably the biggest killer of humans is a disease called cryptococcal meningitis. That's a really big problem for people who have HIV. We think it kills around 100,000 people every year, mostly in sub-Saharan Africa where the burden of HIV is at the highest.

Are there any fungal infections that can spread to the human brain?

Yes, a lot of them can. For example, cryptococcal meningitis. Cryptococcus is a fungus that can cause a pneumonia type of disease in the lung when we inhale its spores. But actually, when most patients present symptoms it's already made it to the brain. We don't quite understand how that happens but we think the fungus gets into the blood and then makes its way up into the brain where it causes significant problems.

A lot of the symptoms you see are typical of meningitis - vision loss, seizures, memory problems. Even people who survive the infection are often left with neurological impairments. Other fungi like Candida yeasts and Aspergillus can also cause significant brain infections in humans, although that tends to only happen in patients who have been left untreated for one reason or another.

How do fungal infections spread?

We mostly get fungal infections from breathing in spores. Most fungal spores are airborne and we breathe them in all the time - every single time we go outside. But it's only when the immune system is damaged that a spore might not get destroyed and germinate inside your lung. Then the fungus can shapeshift into a yeast or a mycelium, a long, thin type of cell, and that's when you get the infection.

There's not much evidence to suggest that once the fungus shapeshifts from a spore into another cell type in your lung that it would go back into spores. So it's not like we're breathing out spores that would then infect other people. There's not much evidence that fungi are infectious diseases like viruses, where if you're near an infected person you might catch it. You tend to get fungal infections from the environment instead.

How do we treat fungal infections?

We actually have very limited numbers of antifungal drugs. So when fungi start causing infections it's hard for us to get rid of them. The reason is that their biochemistry is very similar to ours. If you're trying to make a drug that's going to be toxic to the fungus you have to make sure you're not targeting the same biochemical processes that occur in our cells. That's why we're much more limited in comparison to antibiotics, for example, where we have hundreds of different types.