Video report by ITV News Correspondent Richard Morgan.
A cancer nurse from Newport is urging anyone eligible to get the free flu jab to do so as Public Health Wales warns of a significant threat this winter.
Rowanne Seadon says she sees first- hand how a simple virus can have serious effects on vulnerable patients.
Cases of flu (also known as influenza) are already being detected in Wales and public health experts are warning the flu season could start early and could be as bad as 5 years ago.
During the 2017/18 outbreak around 2,500 people were put in hospital.
Rowanne said anyone who is vulnerable, especially cancer patients and those around them must take the flu vaccine seriously.
"They pick up an infection and are more poorly than most, because they don't have the immune system to fight it off. I strongly encourage those eligible to think about the people in your life who are most vulnerable, and think about getting the vaccine to protect them."
Vaccination is said to be particularly important for those who are older, pregnant, or have health conditions and are more vulnerable to complications as a result of the infections.
Frontline healthcare workers and those who work in care homes or providing care in people's own homes are also being encouraged to receive the flu vaccine to help reduce spread.
As well as that, the Covid-19 autumn booster programme is now live and many people including everyone over 50 years old, and those at risk of severe disease are among those being offered it to reduce their chances of getting seriously ill with Covid-19.
Since March 2020, the coronavirus lockdowns and international travel restrictions have disrupted the amount of respiratory viruses in circulation, but as many people's lives return to normal, viruses are circulating in higher numbers.
This year, Australia's flu season was earlier than usual and saw the highest case levels in five years.
Dr Christopher Johnson, Consultant Epidemiologist and Interim Head of Public Health Wales' Vaccine Preventable Disease Programme is urging all to get the vaccines they need.
"Flu can be serious, particularly for those who are older or have a health condition and are more vulnerable to complications as a result of the flu. It is well known that having a flu vaccine every year is one of the most effective ways to protect against flu.
"Likewise the COVID-19 autumn booster vaccination extends the protection against serious illness. Any side effects from the vaccinations are normally mild and don't last long.
"The chances of becoming seriously ill with Covid-19 or flu are greatly reduced by vaccination, as are the risks of spreading these viruses. Vaccination really is the best way to protect ourselves and others this winter from serious illness."
Eluned Morgan, Minister for Health and Social Services, said: "We are encouraging everyone to get both their jabs this winter, when flu and COVID will be at their peak, to protect themselves, those around them and the NHS during what will be a busy time for healthcare in Wales."
For more information on how to get the flu and Covid-19 vaccines visit the Public Health Wales website.