Geraldine O’Hara says that she became an infectious diseases specialist “because you get to make people better”. And as a registrar at University College hospital in London, she makes people better all the time. “It’s very selfish, but I’ve always found that particularly rewarding. People come in really sick and you make them completely better and then they go home.”

But for five weeks this year, that all changed. She couldn’t cure her patients. She couldn’t stop them dying. Or at least, she could only stop around one in two of them from dying. Because in October, at the height of the outbreak, she volunteered with Médecins Sans Frontières to work in a frontline Ebola clinic in Sierra Leone. She’s 36 years old and as well as her senior position in the NHS, she also conducts research into the immune response to infectious diseases at the London School of Hygiene & Tropical Medicine and has volunteered in sub-Saharan Africa with MSF before. And yet nothing had quite prepared her for the reality of Ebola and its uniquely devastating impact.

“I don’t think anything can prepare you for seeing young people, young people who are completely healthy, and watch them die. One of the first people I looked after was a young guy in his 20s. He was built like a professional footballer. He wasn’t malnourished. He wasn’t someone who was sick beforehand. He was really fit and well. You’d think he must have the physiological reserves to withstand it. And he died. I was taken aback and felt that this is just awful.”

The outbreak first came up on her radar back in January. “Like most infectious diseases specialists, I use a service, ProMED mail, that tracks outbreaks of different diseases around the world. Often, that’s where you’ll get the first inkling of something being up. So we heard about the initial case in Guinea in December and then started to hear about more and more cases.”

It was in April, she says, that she started to realise that something unprecedented was happening. “There were more and more cases but no one seemed to be talking about it. It wasn’t in the press, the WHO weren’t talking about it, there weren’t treatment centres being set up. We just had the feeling that something major was happening and we were all seriously behind the curve.”

It was, she says, incredibly frustrating and when the opportunity to volunteer came up, she jumped at it. “I’ve worked in Somalia, I know about difficult environments and I knew I could help. It felt like quite a privilege, actually. Normally, you see things on TV and you feel distressed and there’s nothing you can do.”

She’d never actually seen a case of Ebola until then. Does it have a certain cachet in the world of infectious diseases? “Yes. When I told my colleagues who are not in infectious diseases, they were like, ‘Oh my God. You’re crazy! What are you doing?’ Whereas my colleagues who are in infectious diseases all said: ‘Oh my God. I’m so jealous!’ I don’t know what kind of light that paints us in, but anyway…”

The work was challenging and often heartbreaking, which came across in a series of radio reports O’Hara did for the Today programme. She spoke about how difficult it was to work in a protective suit when all people can see are your eyes, about living through the experience of watching one of her nursing colleagues discover that she’d contracted it, and the plight of a baby boy who was left in the treatment centre completely alone after his mother died. “No matter what anybody says, I don’t think you can be prepared for the death of children and particularly children on their own…

“I really wanted to convey what it was like. I wanted people to understand what it was like for people there. I didn’t want it to be draining and emotional and shocking. But I did want people to understand that this is happening to normal humans, people like them, and that it’s dreadful.”

She says that she was never worried about catching it. “You’re totally aware of it all the time. You don’t shake hands with anyone, you don’t share food.”

But what about accidentally rubbing your eyes or something?

“Or biting your fingernails, which I do. But it’s not that rugged a virus. Sunlight or a bit of chlorinated water will kill it. It’s relatively easy to deal with.”

It was lack of resources and political willpower that turned it into an epidemic. “It’s made me think a lot about the plague actually. People’s reactions were so extreme. Donald Trump was saying these people shouldn’t come back. You start to think, God, it’s really easy to whip people up into a mob to spread some misinformation… There’s an agenda, certainly in the UK, in politics at the moment about immigrants and disease, which I find a bit frightening. Actually, the last thing you want to do is to criminalise this or make it about blame.”

But she admits her own mother and family were worried. And the experience of seeing so many people die has had an impact she hasn’t quite yet processed. “I’m not the world’s greatest doctor, but even my death rate isn’t 50%.”

Is it going to make Christmas even more special this year, I ask, and she laughs. She used up a lot of her holiday to go to Sierra Leone. “I’m working Christmas Day and Boxing Day. I had to swap some shifts.”

Your poor mum, I say. She waves you off to Sierra Leone and doesn’t even get you back for Christmas. “And I’m hoping to go back out there after Christmas too. It’s quite intense and I have to be a realist because I have a mortgage to pay, but I just really, really enjoy it.”