Staying healthy against viruses is easier if you have a good immune system, according to Dr Michael Mosley.

Famous for creating the 5:2 intermittent fasting diet, the 63-year-old British science journalist has now written a book about stopping the spread of coronavirus.

Dr Mosley began penning COVID 19: What You Need to Know About the Coronavirus in February as the pandemic began to take hold. He described the virus as the extraordinary medical event of his lifetime.

Both his sons Dan and Jack contracted the illness. Dan, having just returned to England from Melbourne, stayed with Dr Mosley and his wife Clare while recovering.

"Dan came down with the infection but was better in a day or so," Dr Mosley explained.

"He was good at self-isolating but my wife and I have wondered if we caught it as we were in close contact with Dan in the 48 hours before he came down with it."

Dr Mosley partly credits his strong immune system for keeping him well against the infection.

"The best way to protect yourself and your family from being infected by the COVID-19 virus is by regular handwashing and social distancing," he said.

"But keeping the virus at bay is going to be really hard in the months ahead.

"The lesson we learnt from the 1918 outbreak of Spanish flu was that the first wave was bad, the second wave was horrendous."

He said until a vaccine is available, it was vitally important to do what you can to support your immune system so it's in good shape when it has to battle a virus.

"One of the reasons why older people are dying in such large numbers is because our immune system tends to become less efficient as we age. It fails to react when it should, then overreacts causing extensive collateral damage.

Dr Mosley said that an ageing and inefficient immune system wasn't inevitable and lifestyle changes could help to keep it in good shape even as we get older.

"One of the most critical things is to lower your weight," he said. "If you're overweight it doubles your risk of being in intensive care and dying. Type 2 diabetes doubles it again."

He said obesity also reduces lung function and affects the effectiveness of the immune system.

"A large waist often goes hand in hand with raised blood sugar levels. People with Type 2 diabetes and pre-diabetes are more prone to getting infections."

Dr Mosley said intermittent fasting diets, such as the 5:2 diet he is famous for, could rapidly reduce weight and bring down blood sugar.

"Sleep is also hugely important for the immune system. If you're sleeping less than six hours per night you're four times more likely of getting a common cold."

According to Dr Mosley, physical activity, managing stress and eating a Mediterranean style diet also boosted the immune system.