Áilín Quinlan asked the experts for their top tips to restore your sparkle. Leftover foods like re-cooked potato and re-heated rice are good for you.
1 There is no magic pill
There's no easy solution to revitalising a flu-battered immune system, says dietitian Paula Mee. "People should understand that in the post-flu period it's crucially important to make a point of eating fruits, vegetables, wholegrains, fish, meat and legumes such as soya beans to help replenish a depleted immune system. You need the full complement of vitamins, minerals and macro-nutrients such as proteins, fats and carbohydrates.
"There's no magic bullet - all foods are important and all of them play a role."
2 Eat Lots of Vitamin C
"It's important to get your immune system back to normal after the flu and a key thing to replenishing it is vitamin C," advises dietitian Sarah Keogh. It's best to do this, she says, by eating lots and lots of good, healthy fruit.
Oranges and kiwis have high levels of vitamin C - while in the vegetable family, says Keogh, broccoli is another good source of this helpful vitamin.
Drinking a small glass of freshly squeezed orange juice is a good idea too, but combine it with the fresh fruit and vegetables in your diet, rather than viewing it as an alternative, Keogh advises.
3 Use Up Your Leftovers
Eat a very diverse range of foods - and this includes leftovers, like potato or heated rice, urges dietitian Paula Mee.
"Our microbiome, or gut bacteria need a very large number of different foods to stay healthy and keep the gut healthy."
Us humans are social creatures, points out Dr Mark Rowe, GP, author of, A Prescription for Happiness and expert in lifestyle medicine. It's really good for us to connect with other people, Dr Rowe explains, because positive, fun social interaction results in the release of oxytocin, a brain hormone which helps strengthen the immune system by reducing inflammation in the body. "It's a good idea to make the time to lighten up and laugh - this also increases the production of oxytocin," he reveals.
5 Yes, Get the Jab
Even if you've had the flu, consider getting the flu vaccine for the sake of your immune system, advises Dr Andrew Jordan GP and Chairman of the National Association of GPs.
"This is because there are a number of different strains of the flu around," he explains.
"Even though you've had one strain of the flu, you're still at risk of becoming ill from another strain - so protect your immune system.
"The importance of the vaccine cannot be over-emphasised because it confers immunity against a number of strains of flu contained in the vaccine."
6 Get Your Iron In
Iron is very important for the immune system and for the body in general, says Sarah Keogh, who warns that in Ireland, studies have found that nearly 50pc of women don't eat enough of the mineral.
The best places to find it are in red meat, particularly beef and lamb, while liver, says Keogh, is a fantastic source of iron - if only Irish people would eat more of it.
"Liver is very nutrient dense and very good for you," she says, adding that chickpeas and lentils are also very high in iron also. Top up your iron levels by adding pumpkin seeds or raisins to your breakfast cereal, she adds.
7 Trash the Ash
Don't smoke following the flu, says Dr Jordan. "Smoking has a negative effect on the immune system," he warns, adding that a really good time to finally kick the habit is following a bad dose.
"Many people reduce the amount they smoke during an illness, such as sore throat or having trouble breathing."
It's worth stopping because smoking has been shown to suppress how your immune system functions and slow down recovery.
8 Avoid Processed Foods
Yes we do love 'em - but it's time to face the fact that those processed foods "hijack" the immune system, warns nutritionist Gaye Godkin.
They induce the production of pro-inflammatory proteins and also disturb your all-important microbiome or gut bacteria, she explains.
"Basically you have to keep the good bugs in the gut well-fed so that they multiply and proliferate and prevent the dominance of nasty bugs."
But processed food doesn't feed the good guys, Godkin explains. And in the long term, this has a negative impact on the health of your immune system, because as we've already explained, the health of your gut has an impact on your immune system.
9 Eat Soya Beans
Soya contains the essential amino acids needed for a healthy immune system, says Paula Mee, who adds that soya beans in particular are an excellent source of this important nutrient.
"Soya has all the essential amino acids required by the human body. Meat, fish and eggs are also good sources of amino acids.
"When you're sick, your reserves of amino acids can diminish somewhat because you appetite may not be as good as it usually is."
We can store fats and carbohydrates, she explains, but we cannot store protein, from which our immune cells are made - and protein is made up of amino acids. "Therefore amino acids found in products such as soya beans as well as in eggs, fish and lean meat are very important to the health of immune system."
So why not drop in to the supermarket, buy some of those healthy frozen soya beans, and start popping them into your homemade soups and casseroles.
10 Invest in Oil
Anti-inflammatory oils such as Omega-3 oil which can be found in olive oil, chia seeds, linseeds and walnuts, protect and support the immune system and control and fight infection, explains Paula Mee.
Take time out to sprinkle some seeds and walnuts on your bowl of porridge every morning. In the evening, try tossing your pasta in a little olive oil. Your immune system will thank you for it.
11 Take Your B Vitamins
The family of B vitamins is absolutely critical to the immune system's ability to manufacture the immune cells which fight infection in the body, explains Sarah Keogh.
Vitamin B is found in oily fish such as sardines, salmon and mackerel - tinned or fresh. And let's not forget that the Irish Heart Foundation recommends eating oily fish up to twice a week.
12 Recharge from Stress
Reducing your stress levels is very important when it comes to re-building a battered immune system, says Dr Rowe.
Meditation, for example, has been shown to be a "tremendous way of boosting the immune system," he explains:
"In as little as 11 hours of meditation, structure changes take place in the brain - the happiness centre of the brain increases as do the areas dealing with awareness, compassion and reflection.
"It damps down the response of the brain's emotional alarm centre, the amygdala, which is hot-wired for anxiety, fear and negative stress.
"You become calmer and your immune system becomes more robust and less fragile."
Dr Rowe also recommends practising to breathe more slowly.
"Slowing your breathing to about five breaths a minute is another great way to build your sense of presence and recharge from stress.
13 Try Probiotics
Probiotics are important to the immune system, primarily because they are good for the microbiome, or the good bacteria, which live in our gut, says Keogh, who observes that one fact we often forget is that the gut is the biggest part of the immune system.
Research has shown that a healthy gut can have a positive effect on the immune system, she says, and taking a good probiotic following a nasty bout of flu - they can easily be found in your local supermarket or pharmacy - helps both the gut and immune system.
14 Restorative leep
Getting sufficient restorative sleep is the foundation for good health, says Dr Rowe. Lack of sleep weakens the immune system and leads to the production of increased levels of stress hormones such as cortisol, he warns.
"Sleep deprivation also increases the levels of inflammation in the body all of which can overload the immune system.
"Research has shown for instance, that a person who has had sufficient sleep - eight hours a night - in the week before getting the flu vaccine has a much better response to it."
We're well aware that exercise is great for everything from weight control to de-stressing and overall strong mental health - but did you know it's also super for the immune system?
"Exercise reduces the inflammation in the body and releases positive neurochemicals such as serotonin, oxytocin, dopamine and natural endorphins," explains Dr Mark Rowe.
"All of these work to strengthen the immune system and reduce inflammation in the body."
Dr Rowe recommends making a point of getting 30 minutes of "green exercise" daily - this means getting out into nature for your exercise.
Ensuring that you have outdoors exercise for 30 minutes a day is also very good for your circadian rhythm and helps regulate your body clock and melatonin levels.
"Healthy melatonin levels dampen inflammation, and this helps strengthen the immune system," he emphasises.
It's also important to remember to move about as much as possible during the day, he adds.
"If you're at a desk, ensure that you get up and move around every 30 minutes or so - even just stand up and go to the water cooler or squat down to pick a biro up from the floor," he urges.
The reason the gut is such an important part of the immune system is because it houses more than 70pc of our immune cells, she explains.
"Therefore make sure to feed your good bacteria with a wide range of foods, including dietary fibre such as resistant starch."
Resistant starch can be found in re-cooked potato as in the traditional Irish potato cake or in carefully re-heated rice.
4 Take Time Out for a Chat and a Laugh