Surprisingly perhaps, Dr Emily Leeming, a dietitian at King’s College London and author of Genius Gut: The Life-Changing Science of Eating for Your Second Brain, thinks this age-old saying isn’t entirely off the mark.

“Apples are a rich source of phytonutrients that are important for your immune system, like vitamin C and antioxidants,” she says. Vitamin C helps your body to produce antibodies and your immune cells to travel to sites of infection, while antioxidants can reduce excess inflammation, helping you bounce back faster if you get sick.

More than that, the fruit is particularly good for maintaining a healthy gut. “Given that 70% of your immune cells live in your gut, a healthy gut microbiome is an important part of supporting your immune system,” says Leeming. She says apples support this system in two ways. First, just one apple contains 100 million microbes, which have been shown to contribute to your gut microbiome, helping it to break down food and make phytonutrients available to the body to use. Second, an average apple contains around 4g of fibre – this is a fuel source for the good bacteria in your gut, which help regulate immune response. (For an even higher fibre fruit try pomegranate, kiwi or raspberries.)

Of course, Leeming says, no one food can stop you getting sick. “A variety of different types of fruit and vegetables provides a range of fibres, nutrients, polyphenols and other antioxidants for your health,” she says. “Regularly eat whole grains, beans, nuts and seeds as they are particularly high in fibre, helping you to reach 30g of fibre a day.”

How long will it take for tweaks to your diet to have a positive impact on your gut microbiome? “As little as three days,” says Leeming. “If you’re looking after your gut health, it is likely to start helping support your immune system fairly rapidly.”