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The illness we commonly know as flu is caused by the influenza virus. There are different strains of the virus, and each year in autumn and winter, one or more new types of virus circulate, spreading among communities and infecting lots of people. This is known as seasonal flu: if you catch a flu-like illness during winter it's likely to be this new, seasonal flu virus.

Symptoms of flu include tosse, dor de garganta, sneezing, high temperature, sweats, aches and pains, headache, and náusea. They can be bad enough that you need bed rest for a few days but healthy people tend to fully recover after around seven days.

The false perception of flu as a nothing more than a mild inconvenience - not helped by the notion of a 'man flu' - belies its potential to wreak havoc. In England alone, flu kills 11,000 people on average each year and is responsible for putting many more in hospital with complications from the virus.

High-risk groups

Some people are at higher risk of such complications, as Dr Samar Mahmood, a GP in Barnsley highlights. "A person's particular vulnerability factor relates to how well their immune system functions," he explains.

"For example, in children, the immune system has not fully developed and in older adults, their immune system tends to have weakened. Older people also tend to have other health problems, which can increase the risk of flu-related complications in itself."

Other people who are more vulnerable include pregnant women, people with a long-term medical condition - like diabetes; coração or lung disease; kidney problems; and neurological conditions - and people whose immune system doesn't work properly.

That could be because of illness, like teste de HIV for example, or because of medical treatment, such as quimioterapia or immunosuppressant drugs following an organ transplant.

People with asma or chronic obstructive pulmonary disease (DPOC) may find that a bout of the flu triggers an exacerbation of their condition, making their usual symptoms worse.

For anyone, but particularly people with respiratory illnesses, the most common complication from flu is a bacterial chest infection, which can develop on top of the flu virus itself. In some cases, a secondary chest infection can worsen and lead to pneumonia, where the lung tissue becomes inflamed. This can be serious or even fatal, particularly for the elderly and those with lung disease.

Flu can also cause sinus e infecções de ouvido, and less commonly, the virus travels in the bloodstream to the brain where it causes inflammation, known as encefalite.

"It's difficult to quantify, but we know that it's quite common for vulnerable people to become seriously ill with flu," says Dr Mahmood. "That's why we have the vaccination programme - because of the high prevalence of flu in the clinically vulnerable group."

This year, it's more important than ever that you have the flu vaccination if you're susceptible to complications, because of the added danger of COVID-19.

The viruses present with similar symptoms, making it difficult in some cases to distinguish between them. A new, continuous cough, a high temperature, and a change to your sense of taste or smell indicate COVID-19. If you have these symptoms, you should isolate at home. Use the NHS 111 coronavirus checker to find out what to do next.

You can have both flu and coronavirus at the same time, which can be a perilous combination - especially if you're at high risk.

In fact, during the early stages of the coronavirus pandemic, which overlapped with the end of the 2019-20 flu season, people who contracted both viruses simultaneously were more at risk of serious illness, and more than twice as likely to die, compared with people who only had COVID-19.

The chances are that if you catch flu, the virus will be self-limiting and you'll be able to treat your symptoms at home.

However, it's crucial that you get medical advice if you're clinically vulnerable and you think you have flu. Essentially, you are classed as clinically vulnerable if you are normally entitled to the free NHS flu jab.

Carers and those in regular close contact with people on this list may also be eligible, according to the full criteria.

If you're normally healthy and not clinically vulnerable, you should also seek medical advice if you're still unwell with flu after seven days - although it's worth bearing in mind that cough associated with flu can sometimes persist for a few weeks.

Be on the lookout, too, for signs that you're developing a secondary chest infection. They include recurring fever, worsening cough, shortness of breath, chest pain and tightness in your chest, and fast breathing.

It's important that you don't visit a pharmacy or your GP practice, because even if you're certain you don't have COVID-19 (and the only reliable way of knowing for sure is to have a test), if you're unwell with another flu-like illness you could be more prone to picking up coronavirus from someone else.

Whether or not you're classed as vulnerable, if you're worried at all about your symptoms, seek advice by visiting 111.nhs.uk or calling 111. An adviser will be able to tell you what to do.

The best way of keeping yourself and others safe from flu and its potential complications is to have the annual flu vaccination if you're eligible. This year, even more people can get a free flu jab. And those who aren't eligible for a free vaccine can pay for a vaccination from their pharmacy, which normally costs less than £15.

It's also a good idea to keep viruses (including flu and COVID-19) at bay by washing your hands regularly with soap and water for at least 20 seconds each time - especially when you come indoors after being out, and before and after preparing food and eating.

As well as that, Dr Mahmood says diet is important in keeping your immune system in tip-top condition. "Maintain a healthy diet rich in vitamins C e D in particular," he says. "Supplements are not essential if you have a balanced diet, but they're unlikely to do any harm, as they are, after all, a supplement and not a replacement."

You can treat flu symptoms at home with over-the-counter remedies, like paracetamol, and drinking plenty of fluids. With any luck you'll be back to full health within a week or two.