This week, Professor Luke O’Neill turns his microscope to a condition he knows all too well — cellulitis.

It’s one of the most common bacterial infections there is, affecting about one in every 500 people each year. The skin turns red, swollen, and sore, and you might even feel flu-ish as your immune system fights back. Left untreated, it can spiral into sepsis, making early diagnosis vital.

Listener Joanie suggested this week’s theme, and you can do the same by emailing Luke at laoneill@tcd.ie. As Luke explains, cellulitis happens when bacteria slip through tiny breaks in the skin — a scratch, a cut, even an insect bite — and start multiplying. He suspects that’s how he picked it up himself earlier this year.

Rates of cellulitis are on the rise, with hospitals seeing more admissions, especially in settings where people live in close quarters: nursing homes, homeless shelters, and college dorms. Obesity and antibiotic resistance may also be fuelling the increase.

Treatment is straightforward (antibiotics usually clear it up) but prevention is even better. Keeping skin clean and moisturised, and treating any breaks promptly, can stop it taking hold.

And it’s not just humans. Cats, dogs and even horses can suffer from cellulitis, with vets watching closely for swelling and lameness. In fact, that old-fashioned poultice once used to “draw out” infection still has its place — at least in the stable.

From microbes to moisturiser, Luke explores how our skin keeps the world out. And what happens when it can’t.

This week, Show Me The Science gets a little darker — and a little stranger. Professor Luke O’Neill dives into the science of Hallowe’en, tracing its roots back to the Celtic festival of Samhain, the ancient Irish celebration marking the start of winter — that “dead time” when the veil between worlds was said to thin.

It’s not about ghosts and ghouls, Luke explains, but about remembering ancestors, lighting bonfires, and wearing masks to mark the turning of the seasons. Around those fires, people bonded, built resilience, and reaffirmed community — something modern psychology still recognises as vital in times of darkness and uncertainty.

Luke explores the science of fear: how the body reacts when startled, from adrenaline spikes to endorphin release, and why a good fright, once we recover from it, can actually make us stronger. Researchers have even measured these effects in saliva! But don’t expect Luke to be queueing up for a horror marathon — he admits he doesn’t get the same thrill others do from being scared senseless.

There’s also a scientific story behind our favourite monsters. The myths of vampires and werewolves, Luke reveals, may have stemmed from a rare inherited condition called porphyria, which causes sensitivity to light, red-stained teeth, and even hair growth and behavioural changes — symptoms that once seemed supernatural. And diseases like plague or tuberculosis, which sometimes left victims appearing to rise from the dead, helped fuel the legend of the undead.

From Bram Stoker’s Dracula — possibly inspired by both the actor Henry Irving and the medical realities of syphilis — to the theory that “Dracula” could derive from the Irish “droch fhola,” or “bad blood,” Luke connects folklore, medicine, and etymology in classic Show Me The Science style.

And there’s even a royal twist: King George III famously suffered from porphyria, and through royal intermarriage, the disease spread across Europe’s bloodlines — adding a touch of genetic truth to the vampire myth.

This special Hallowe’en edition was produced in collaboration with the Music Technology course at South East Technological University. The haunting original music was composed and recorded by Billy Doran.

Ever wondered what keeps your immune system in check when it starts misbehaving? On this week’s podcast, Professor Luke O’Neill explains the world of regulatory T cells—the immune system’s very own “military police.” These tiny guardians stop other cells from attacking the body, helping prevent autoimmune diseases and keeping us healthy.

Luke takes us through the Nobel Prize-winning discoveries of Mary Brunkow, Fred Ramsdell, and Shimon Sakaguchi, who cracked the secrets of immune tolerance. Back in 1995, Sakaguchi discovered these remarkable regulatory T cells, proving that the immune system is far more sophisticated than previously thought. Brunkow and Ramsdell uncovered the critical role of the Foxp3 gene in autoimmune diseases, and Sakaguchi later linked the Foxp3 gene to the development of these life-saving cells. Luke even shares insights from a recent encounter with Sakaguchi himself!

If you’ve ever been curious about how the body avoids attacking itself, or how groundbreaking immunology research leads to real-world insights, this episode is your backstage pass to the science of regulatory T cells.

Got questions, ideas, or thoughts for Luke? Email him directly at laoneill@tcd.ie