Influenza (the flu) is a viral infection of the respiratory tract that spreads easily and quickly from person to person. Symptoms include fever, headaches, muscle aches, and tiredness. Treatment for flu is mainly to lessen symptoms and provide comfort. Vaccination is the best means of prevention. People with immature or weakened immune systems are at a higher risk of developing serious complications from flu, such as pneumonia and bronchitis, but it can rarely seriously affect any individual.
In New Zealand, approximately one in four people are infected with flu each year. Most cases occur during the winter (that is, May to October). Although many of these people will not develop symptoms, they can still pass the flu on to other people, which is why vaccination is important.
What is flu?
The flu is an infectious disease of the respiratory tract – the nose, throat, and lungs, caused specifically by different strains (variations) of influenza viruses. The main virus strains undergo genetic change (mutation) allowing them to evade our immune systems, which is why people need to get a new vaccination every year to have immunity.
The annual flu strains that circulate every winter generally do not cause widespread illness. Sometimes, however, flu outbreaks occur in which many people are affected at the same time. These outbreaks are called epidemics if they occur in one location, while worldwide outbreaks are called pandemics.
What is the difference between a cold and flu?
Colds and flu are both infectious respiratory diseases and share some of the same symptoms (such as coughing, sore throat, and runny nose) so it can be difficult to tell the difference. The main distinction is that flu is a more serious illness, resulting in symptoms being felt with greater intensity and carrying greater risk of serious complications and sometimes death. Also, colds and flu are caused by different viruses.
Duration of the illness is a further point of difference: cold symptoms can make you feel ill for a few days, while flu symptoms can make you feel unwell for a few days or up to two weeks. And, unlike flu, there is no vaccine to protect against the common cold and no antiviral drugs have been developed to effectively treat the common cold.
Symptoms and diagnosis
Symptoms of flu, which can come on suddenly, include the following:
- Fever and chills
- Cough, sore throat, and a runny or stuffy nose
- Body aches and pain
- Weakness and tiredness, sometimes severe
- Irritated watering eyes.
Nausea, vomiting, and diarrhoea may also occur during a flu infection, especially in children. Most people who get the flu have symptoms for one to two weeks and then recover without any problems. However, people with weakened immune systems can develop serious and potentially life-threatening complications, such as pneumonia and bronchitis.
Diagnosis of flu is based on symptoms and a person’s association with other people known to have the disease. A rapid diagnostic test, by taking a swab sample from the upper part of the throat, can be done in a doctor’s office to determine the particular influenza virus strain.
Some of the symptoms of flu and COVID-19 (coronavirus disease 2019) are similar, which can make it hard to tell the difference between the two diseases based on symptoms alone. Diagnostic testing can help determine if a person is sick with flu or COVID-19.
Treatment for flu is mainly focused on alleviating symptoms and making the person as comfortable as possible. This includes getting plenty of bed rest at home (away from other people) and drinking plenty of fluids, using a damp cloth on the forehead to reduce fever, and taking paracetamol to relieve muscle aches and fever.
People at higher risk – the elderly, young children, pregnant women, and those with existing chronic medical conditions (including asthma, diabetes, and kidney disease) – should see their doctor early, to find out if they need treatment. Antiviral drugs are available on prescription to reduce the severity of symptoms and prevent complications. They work best when taken within the first two days of symptoms appearing.
Medical advice should also be sought early if you have concerns that the illness could be a more serious condition, such as meningococcal meningitis which is similar to flu in its early stages.
The illness and death caused by seasonal flu can be reduced in frequency by getting an annual influenza vaccination (the ‘flu jab’).
Seasonal vaccination is especially recommended for those people who are in an at-risk group. People aged 65 years or over, pregnant women, or people that have a health condition such as diabetes or heart disease that puts them at greater risk of influenza, can get the influenza vaccine free at a general practice or pharmacy that offers vaccinations.
As part of New Zealand’s response to COVID-19, vaccination of those at highest risk of influenza is being prioritised. Although influenza vaccination does not prevent COVID-19, influenza causes thousands of hospital admissions and doctor's visits. Reducing the number of cases of influenza will help to improve the ability to of the healthcare system to cope with any increased demand due to COVID-19.
If you develop flu symptoms, you should follow basic hygiene practices to avoid spreading the disease to other people, including:
- Washing your hands regularly and thoroughly (with soap and warm water for at least 20 seconds and drying for 20 seconds) or use an alcohol-based hand rub
- Avoiding touching your eyes, nose, and mouth
- Not sharing eating or drinking utensils
- Covering your mouth and nose with a disposable tissue or the inside of your elbow when you cough or sneeze
- Staying away from work or school for as long as you have symptoms.
Further information and flu tracking
Healthline is a free Ministry of Health service for health advice and information, which is staffed by experienced healthcare professionals.
Freephone (24 hours): 0800 611 116
FluTracking (https://info.flutracking.net/) is an online health surveillance system used to detect the potential spread of influenza. The survey is sent via email each week to registered participants, asking if you have had fever or a cough, or other flu-like symptoms in the last week. The data obtained is used to help track the spread of influenza-like-illness and COVID-19 in New Zealand, and to provide early warning of possible outbreaks and monitor trends during pandemics.
Centers for Disease Control and Prevention (2020). Cold Versus Flu (Web Page). Atlanta, GA: Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases. https://www.cdc.gov/flu/symptoms/coldflu.htm [Accessed 07/02/21]
Immunisation Advisory Centre (2020). Influenza (Web Page). Auckland: The Immunisation Advisory Centre. https://www.immune.org.nz/diseases/influenza [Accessed: 07/02/21]
Ministry of Health (2021). Influenza (Web Page). Wellington: New Zealand Government Ministry of Health. https://www.health.govt.nz/your-health/conditions-and-treatments/diseases-and-illnesses/influenza [Accessed 07/02/21]
Nguyen, H.H. (2020). Influenza (Web Page). Medscape Drugs and Diseases. New York, NY: WebMD LLC. https://emedicine.medscape.com/article/219557-overview [Accessed: 07/02/21]
O’Toole, M.T. (Ed.) (2017). Influenza. Mosby’s Dictionary of Medicine, Nursing & Health Professions (10th ed.). St Louis. MI: Elsevier.
Reviewed: February 2022
Go to our Medical Library Index Page to find information on other medical conditions.