Every now and then one pops up at work, in the pub, at the park, outside the school gate or in your own family - the person who claims to never get sick.
Colds brush past such people without leaving so much as a sniffle. They never seem to take a day off work.
They appear to be superhuman, with the kind of immune systems the rest of us mere ailing mortals can only dream about.
What are their secrets? Do such people even exist?
"I hardly ever get a cold, bug or infection," declared Ms Lore Lucas, a 97-year-old Jewish refugee and Holocaust survivor who has lived in Glasgow since 1946.
"I've never drank or smoked. I sleep well and I like a little rest during the day."
What about her diet?
Some of us inherit a set of immune system genes that are good at dealing with one particular virus.
PROFESSOR DANIEL DAVIS, an immunology specialist at the University of Manchester.
"I've been known to have a great dislike for cheese and I really do not like Scottish specialities like mince, haggis or porridge," she said.
During her working life, first as a maternity nurse in Geneva, where she lived after fleeing Nazi Germany in 1938, and then as an office secretary, she had never taken a day off due to sickness.
Ms Lucas has been a widow for 30 years and has one son and a granddaughter. She puts her exceptional health down to a combination of good genes and a good life.
Oh, and a good game of bridge.
"I play in various clubs and also enjoy a social game at home. I am quite addicted," she confessed.
On average, each of us will get around 200 colds in a lifetime.
Goodbye, MC King... really?
•Be realistic: There is no such thing as not getting sick at all.
•Don't smoke: And also don't drink too much alcohol.
•Wash your hands regularly: But remember that infections are mostly passed on through proximity. "If you want to avoid a person who has cold symptoms, you are better off moving to another carriage on the train than using hand sanitiser," said general practitioner Ann Robinson.
•Exercise regularly and moderately: And also remember to rest. There is evidence that regular exercise, which improves circulation, can boost immunity.
•Manage stress: "The best established link in terms of how lifestyle impacts the immune system is that stress levels relate to your immune system's behaviour," said Professor Daniel Davis of the University of Manchester. Chronic long-term stress produces cortisol, which neutralises immune cells.
•Immunise: If you are likely to be at increased risk of infection, whether through chemotherapy, long-term steroid use or pregnancy, get yourself vaccinated.
•Maintain a healthy and varied diet: But don't go overboard. This connects to the latest research on the importance of the gut microbiome. "A lot of the chemicals important to our immune system originate in the gut," said Dr Robinson.
•Sleep well: "Sleep has a massive impact on the immune system," said Dr Natalie Riddell, a lecturer in immunology at the University of Surrey. It is under the control of circadian rhythms, she added, and disturbing it can upset your immune system.
•Stay connected: "If there is one thing that's the enemy of wellbeing, it's loneliness," said Dr Robinson. "Get out there and connect with people, if not with their viruses."
Though some people appear to suffer more than others, there is no evidence or research on why, or if, that is really the case.
Dr Natalie Riddell, a lecturer in immunology at the University of Surrey, said: "It's pretty much hearsay and self-reporting. I need more evidence before I can believe these people really exist."
Though there is no scientifically- proven link between lifestyle and enhanced immune function, the immune-boosting industry continues to flourish.
Nutritional supplements are expected to be worth US$60 billion (S$85 billion) by 2021.
Meanwhile, for doctors and immunologists, the notion of superhuman health remains at best unproven and, at worst, a fiction.
This is because of the individual and complex nature of our immune systems, which are almost as specific to each of us as our fingerprints.
"Some of us inherit a set of immune system genes that are good at dealing with one particular virus," said Professor Daniel Davis, an immunology specialist at the University of Manchester.
"But that is not to say that you or I would have a better or worse immune system. All it means is that you would deal with a particular flu virus better than me," he said.
"There is an inherent diversity in how our immune systems respond to different diseases and that diversity is essential to how our species survives disease."
Much of this diversity comes down to our genetic make-up.
Prof Davis said: "The greatest diversity in all of the 25,000 genes that make up the human genome is in our few immune system genes."
It throws into question the benefits of products claiming to boost our immunity - antioxidants, vitamin C, hot lemon and ginger tea, garlic, echinacea or wheatgrass. Do any of them work?
"The bottom line is that we simply don't know," Prof Davis said.
So why do some people seem to be better at fighting infection?
General practitioner Ann Robinson said: "Maybe people at the top end have been primed through early exposure to bugs, being fully vaccinated and so on."
Prof Davis added: "Each person is wired to be slightly better at fighting off some illnesses and slightly worse at fighting off others."
Both experts pointed to evidence that our gut microbiome, the range and quantity of microbes in our guts, impacts the immune system.
So is there a link between diet and immunity?
Prof Davis said: "Although gut microbiome directly affects the immune system, precisely how isn't yet clear."