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Explainer: How the human body fights infection and how you can boost your immunity

SINGAPORE — While scientists continue to learn more about the Covid-19 coronavirus, a question that is often asked is this: Why do some people die from an infection but then there are others such as Singapore’s 76th patient — a one-year-old baby who recovered in just two days — who have mild or no symptoms?

Explainer: How the human body fights infection and how you can boost your immunity

SINGAPORE — While scientists continue to learn more about the Covid-19 coronavirus, a question that is often asked is this: Why do some people die from an infection but then there are others such as Singapore’s 76th patient — a one-year-old baby who recovered in just two days — who have mild or no symptoms?   

The answer, experts said, might possibly hinge on a person's immune defences.

Researchers are still unclear exactly how this new coronavirus interacts with the human body's immune system, but from reports that have come up, around 80 per cent of the infected cases recover but the rest grapple with severe or critical symptoms.

An estimated 2.3 per cent die from it, based a new study published in the Chinese Journal of Epidemiology that looked at more than 72,000 cases of Covid-19 infection in China as of Feb 11. 

The study by the Chinese Centre for Disease Control and Prevention showed that for those aged 80 and above, the fatality rate is harrowing — at 14.8 per cent.


Our immune system kicks in when it encounters a threat, such as a virus, bacteria or parasite. Yet, it is common to see differing immune responses to the same microbe — even among family members.

Paediatric immunology specialist Liew Woei Kang is the medical director of the Paediatric Allergy Immunology Rheumatology Centre at Mount Elizabeth Novena Hospital. He said that there is an interplay between a person's genes and environmental factors that determine the immune system and its response to infection.

This means that lifestyle factors such as how you live, what you eat, how much exercise you get and your medical history could possibly make a difference between life and death in the face of an infection.

The immune system is made up of a complex network of organs such as the spleen and lymph nodes, immune cells as well as molecules that can be activated to defend against threats to the body.

Associate Professor Liu Haiyan said that the body’s first line of defence in response to an infection is the barrier function, which includes the skin and mucosal surfaces — the inner lining of some organs and body cavities such as the nose, mouth, lungs, stomach and intestines.

Assoc Prof Liu is from the department of microbiology and immunology at the Yong Loo Lin School of Medicine in the National University of Singapore.

Like many respiratory infections, Covid-19 is believed to be spread mostly through droplets emitted from an infected person, such as when the person coughs or sneezes.

Someone may become infected if the droplets come into contact with the mucous membranes in the eyes, nose or mouth. Transmission may also occur when people touch their eyes, nose or mouth by hands exposed to the virus.

This is also why medical experts have been saying that good personal hygiene is important in preventing infections, because viruses would have to get into the body to cause disease.

The immune system then initiates an early response (called innate immune response), during which immune cells can be activated within a short period of time. However, this early response is usually not enough to deal with an infection, Assoc Prof Liu said.

“We need the activation of our adaptive immune response, which takes about three days (after the early response is initiated).

“Lymphocytes (one of the types of white blood cells) are the main cell types responding at this stage. Some of them can kill viral infected cells and some can make antibodies that help clear the infection.”


Assoc Prof Liu said that there are no known medications or vaccine for new pathogens such as Covid-19 for now, so people would heavily depend on their immune systems to fight the infection.

She defines a healthy immune system as one that is “balanced and targeted” and can generate a specific response to infections. And it does this without inducing much damage to the body’s own tissues and organs.

“People who can generate a more efficient anti-viral immune response have milder symptoms when they encounter an infection,” she said.

She explained that the immune system starts to function from birth but also goes through different stages in a lifetime.

Expanding on this, Dr Liew said: “At birth, babies are partly protected by antibodies from their mothers, transferred via the placenta in the last few weeks of pregnancy. Premature babies have lower immunity levels."

After birth, the immune system “gets educated” by encounters with various microbes, including infections, probiotics and vaccinations, he said.

The immune system is possibly most primed — or ready for “battle” — in childhood and then matures in adulthood, he added.


If a young immune system is not fully matured, why do children seem to have less severe forms of Covid-19 infection?

Associate Professor Thoon Koh Cheng of KK Women’s and Children’s Hospital (KKH) said that the effects of the new coronavirus on children seems to share some similarities with the one that caused the severe acute respiratory syndrome (Sars) in 2003.

“In our previous experience with Sars, patients who are 12 years of age or younger had milder disease. The younger children were also less likely to be admitted to an intensive care unit or receive supplemental oxygen.”

Assoc Prof Thoon, who is head and senior consultant of infectious disease service at the hospital’s department of paediatrics, said that based on what doctors know so far, if the virus infects a healthy child, he or she should make a good recovery with good medical support.

“Milder symptoms are also observed in children infected with the coronavirus compared to older people.”

Dr Liew offered several possible hypotheses for this. One of them may be due to children’s many recent encounters with infections including the common human coronavirus, which offer some cross-protection.

For instance, it is common for children to get one to two infections a month in the first six months of starting school — due to a combination of poor hygiene habits (such as children sneezing and coughing into one another) and viruses circulating within air-conditioned confined spaces, he said.

There are several common human coronaviruses already known that cause symptoms such runny nose, sore throat, headache, fever and cough.

Dr Liew explained: “While a child’s immune system is young, the ‘soldiers’ are ready. In contrast, an adult’s immune system may have experience with previous infection encounters, but because the last illness may have been a while back, the immune system may not be as battle-ready.”

An adult’s immune system may also be senescent (ageing) or dysregulated (not functioning normally), causing an exaggerated immune response.

“The severe Covid-19 cases in intensive care and deaths are often not due to the virus, but an exaggerated response to the virus. This results in excessive inflammation in the lungs,” Dr Liew said.


The immune system slowly declines with age and becomes less efficient in dealing with new infections. This is why old people are vulnerable even to common infections.

Furthermore, adults usually also have other medical conditions such as ischaemic heart disease or diabetes, which puts them at a higher risk of complications from infections, Dr Liew said.

Assoc Prof Liu said that as a person ages, there could be a reduced number of infection-fighting lymphocytes that respond to the new infection, or pre-existing conditions that dysregulate immune responses. However, it is impossible to specify an age in which the decline happens. 

Other factors such as the person’s lifestyle habits, nutrition and mental health status matter, too.

Dr Melvin Look, a consultant gastrointestinal and laparoscopic surgeon at PanAsia Surgery, pointed out that old people tend to also have micronutrient malnutrition, which can affect their ability to fend off infections.  

At a media talk on gut health and immunity on Tuesday (Feb 18), Dr Look said that research has found links between the immune system and the gut, which contains 70 per cent of the cells of an immune system.

“The immune system requires a healthy gut to provide nourishment,” he said.

“With ageing, the gut is less able to absorb micronutrients from food consumed. Because of the lack of these essential micronutrients, the immune system becomes less efficient.”

There is also evidence to show that mental health has a big impact on physical health, Dr Look added.

Psychological stress of any duration, from several days to the chronic types that lasts for years, can cause immunity to go downhill.

In some cases, the immune system may also go awry, and over-react to perceived threats when it should not.

Assoc Prof Liu said: “If our immune system is over-reactive to some environmental agents that are not harmful to us, such as dust mite and pollen, we can have allergies.

"Autoimmune diseases are caused by an activated immune system targeting our own tissues, resulting in inflammation and damage.”


While some factors such as genetics or age cannot be changed, the medical experts said that making certain lifestyle changes might be able to improve a person’s response to illness.  

“People who have a healthy lifestyle with regular exercise, balanced nutrition and less stress tend to have a more efficient immune system fighting infections,” Assoc Prof Liu said.

Support your immune system with the following tips:

1. Cut out junk food

Dr Look said that processed food — refined carbohydrates, sugar, processed meats, fat — disrupts both the gut and immune system.

“The first thing you should do at a time when you might be prone to infections is to stop taking so much sugar. This is because bacteria, viruses thrive on sugar. The more sugar you keep, the easier it is for them to gain a foothold on your defences,” he said.  

2. Ensure a well-balanced diet with nutritious foods

Dr Liew, who sees more infections in fussy eaters with a low vegetable and fruit intake in his practice, emphasised the importance of a balanced diet with adequate water intake. He said that vitamin C supplementation has the strongest evidence in reducing recurrent infections.

Dr Look advised increasing the intake of plant-based whole foods that are packed with micronutrients such as zinc, iron, copper, folic acid and vitamins A, B6, C and E. These are required for the body to fight infections.

The general rule of thumb for healthy eating is to “follow the rainbow”, he said. Eating a wide range of vibrantly coloured fruit and vegetables provide a variety of nutrients needed for good health. These may include vegetables such as tomatoes, berries, pumpkin, oranges, spinach, eggplant and broccoli.

3. Consider intermittent fasting

Dr Look said it has been proven that intermittent fasting, a form of time-restricted fasting, promotes autophagy. This is a process whereby the white blood cells clean out the unhealthy, damaged cells in the body.

There is no hard and fast rule as to how often one should fast, but Dr Look said that beginners can try a 16-hour fasting, followed by consuming food during an eight-hour window, several times a week.

All other rules of healthy eating and maintaining adequate nutrition still apply when it is time to eat.

4. Get regular doses of sunshine or Vitamin D

A person who is deficient in Vitamin D is likely to be more susceptible to infections.

Vitamin D, which is produced by the body when exposed to sunlight and in small quantities in food such as salmon, egg yolk, fortified orange juice and cereal, could boost the immune system, Dr Look said.

Exposure to sunshine also elevates mood and improves sleep at night, which are also important in maintaining a good immune system.

The World Health Organization states that getting five to 15 minutes of sunlight on the arms, hands and face twice or thrice a week is sufficient.

5. Exercise regularly, but do not overdo it

One of the basic tenets of healthy living, regular exercise has been linked to fewer colds and infections.

A study published in the American Journal of Lifestyle Medicine in 2011 found that moderate exercise of walking improved immune response and reduced incidence of upper respiratory illness.

People who engaged in regular moderate exercise — such as five days a week of walking 45 minutes each session over 15 weeks — reduced their number of sick days by up to half, compared to sedentary people.

However, too much intense exercise, such as more than 90 minutes of high-intensity endurance training, has been linked to reduced immunity.

6. Manage stress

Numerous studies have shown the relationship between physical and mental health. Stressed out, isolated people tend to have poorer immune functions, the American Psychological Association said, and recommended stress-relieving tips such as:

  • Adopting good lifestyle, sleep and dietary habits

  • Cultivating strong social support to improve resilience

  • Relaxing muscles through stretching, massages or warm baths

  • Meditating for five minutes a day or similar calming activities

  • Taking a moment to notice nature

  • Reframing thinking to manage emotions

  • Seeking professional help

7. Take protective measures 

Even if your immune system is in tip-top condition, taking steps to avoid infection is still important in preventing infections. This is because viruses would have to get into the body to cause disease, Assoc Prof Liu said.

Assoc Prof Thoon of KKH said that while there are very few cases of Covid-19 infection reported among children, people of any age are susceptible to infection.

Here, again, are the necessary protective measures against this new coronavirus:

  • Avoid close contact with people who are unwell or showing symptoms of illness

  • Observe good personal hygiene

  • Wash hands with soap and do not touch the eyes, nose or mouth after touching surfaces that are commonly used

  • Cover the mouth with a tissue paper when coughing or sneezing and dispose of the soiled tissue paper in the rubbish bin immediately

  • Seek medical attention promptly if feeling unwell and wear a mask

  • Children who are too young to comply with strict hand hygiene should be guided by parents and caregivers

  • Avoid crowded places

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