Why does everyone get sick so often in winter?

It’s not your imagination, people do get sick more often in the colder months. Now a doctor has revealed the real reason why this happens.

Welcome to Ask Doctor Zac, a weekly column from news.com.au. This week, Dr Zac Turner explores why so many people get sick in the winter.

QUESTION: Hi Dr Zac, Every winter like clockwork, my sharehouse becomes a snotty, coughing mess. I always assumed it was just because me and my flatties are inside more, but I read something about viruses surviving longer in colder temperatures. Is that true? And does the lack of sun during winter actually mess with our immune systems? – Matt, 29, Ballarat VIC

ANSWER: Thanks for the question, Matt – and condolences to your household’s sinuses.

Winter really does bring with it a revolving door of colds, flus and respiratory infections. It’s not your imagination. And while yes, spending more time indoors definitely helps viruses spread, the science of why we get sicker in winter runs a bit deeper – and chillier – than that.

Let’s start with the temperature. Many common viruses, including rhinoviruses (the ones behind most colds) and influenza, actually survive longer and replicate more effectively in colder, drier conditions. One study from the Journal of Virology found that some viruses become more stable and infectious in lower temperatures because the cold preserves the viral envelope – essentially giving them a longer shelf life while they search for their next victim (you).

Add in the fact that we’re all breathing the same stale air indoors, and it’s a perfect storm: less ventilation, more close contact, and viruses that are thriving in the cold.

Some viruses become more stable and infectious in lower temperatures. Picture: iStock

Now, let’s talk about vitamin D. This isn’t some wellness influencer’s pet obsession – it’s genuinely important for your immune function. Vitamin D helps regulate immune responses, and deficiency has been linked to increased susceptibility to infections, particularly respiratory ones.

In winter, shorter daylight hours and less skin exposure to UVB rays means your body produces less vitamin D naturally. In fact, data from the Australian Bureau of Statistics shows nearly one in four Aussies have insufficient vitamin D during the colder months – especially in southern regions like Victoria and Tasmania.

So yes, Matt: cold temperatures, indoor living, and low vitamin D levels all team up to make winter a bit of a microbial minefield.

Why do people get sick more often in winter? Picture: iStock

What can you do about it?

• Maximise ventilation: Even cracking a window makes a difference. Let your house breathe.

Get outside during daylight hours: A lunchtime walk could help top up your vitamin D – just 10–15 minutes of sun exposure to arms and face can help.

• Eat immune-supporting foods: Fatty fish, eggs, mushrooms and fortified milks are great sources of vitamin D. And yes, supplements are helpful if you’re deficient – but check with your GP before starting.

• Practice good hygiene: Wash hands, cover coughs, and maybe don’t share that water bottle with your sniffling housemate.

Go on, eat the fish. Picture: iStock

A few smart habits can help make your winter less snotty – and keep your immune system in fighting form until the sun comes back.

Stay warm,

Dr Zac

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Dr Zac Turner is a medical practitioner specialising in preventative health and wellness. He has four health/medical degrees – Bachelor of Medicine/Bachelor of Surgery at the University of Sydney, Bachelor of Nursing at Central Queensland University, and Bachelor of Biomedical Science at the University of the Sunshine Coast. He is a registrar for the Australian College of Rural and Remote Medicine, and is completing a PhD in Biomedical Engineering (UNSW). Dr Zac is the medical director for his own holistic wellness medical clinics throughout Australia, Concierge Doctors.

Originally published as Why does everyone get sick so often in winter?