Clinical description and diagnosis

Page last updated: 19 January 2011


Influenza is often confused with the common cold. They are both respiratory illnesses but are caused by different viruses. While there are similar symptoms, influenza is more severe and tends to make your whole body ache, whereas a cold is milder, comes on slowly and usually only affects your nose and throat. If you have a cold, you can usually carry on with day-to-day activities. If you have Influenza you feel very sick and want to stay in bed.

A person usually develops symptoms of the Influenza within one to three days after becoming infected with the virus.

Symptoms of Influenza:

  • a fever (temperature over 38C) that often appears suddenly and may be associated with feeling chilled and shivering
  • headache
  • muscle aches and pains
  • sore throat
  • dry cough
  • trouble breathing
  • sneezing
  • stuffy or runny nose
  • tiredness
  • loss of appetite.
The fever usually subsides in 3 to 5 days, and the person begins to feel better. The tiredness and cough may, however, persist for a few weeks. Sometimes, usually in young children and the elderly, influenza can be associated with vomiting and diarrhoea.

Older children and teenagers experience the same symptoms of influenza as adults. Very young children and infants probably experience similar symptoms but may not be able to tell people they have sore muscles or a headache. They may be irritable, have no appetite, and sometimes develop a hoarse cry and barking cough (like croup).

If you or members of your household experience any of these symptoms, avoid contact with other people wherever possible. If you are concerned about the symptoms, or they become worse, seek medical advice immediately. It is especially important to get medical advice at the first signs of illness in children and in people who have certain chronic medical conditions. Your doctor will be able to advise if you have a chronic condition which may put you at particular risk of the complications of influenza

Top of Page

Influenza can spread easily

Adults can spread the seasonal flu virus for up to 7 days, primary school aged children for up to 14 days, and pre-school aged children for up to 21 days. People are most infectious when they are still coughing and sneezing. In some people, the contagious period can begin the day before symptoms develop.

The main ways in which the influenza virus can enter a person’s body and cause disease are via:
  • Respiratory spread – when an infected person exhales, their respiratory droplets can spread into the eyes, nose and mouth of an uninfected person. The uninfected person needs to be relatively close by - usually within a distance of around a metre.
  • Contact spread - if an uninfected person has virus on their hands and they touch their own eyes, nose or mouth they can infect themselves. A person's hands may be contaminated by touching nasal fluids such as wiping a child’s nose, used tissues, doorknobs or other items or surfaces that an infectious person has contaminated.
As the virus can not survive for long periods of time in the environment, contamination of hands is most likely to occur following contact with freshly soiled items.

Top of Page

Future Pandemic Influenza strains

Pandemic influenza is caused by a subtype of the influenza A virus that has not circulated in the human population for some time, and is capable of causing severe disease and spreading easily from human to human. To do this the virus must change from one that only occasionally infects humans to one that is easily spread between people.

The pandemic influenza virus cannot be fully understood until the virus strain emerges. For planning purposes it is assumed that a pandemic strain of influenza will have similar transmission and
communicability as seasonal influenza. This website will be updated as new information about the clinical and epidemiological characteristics of the disease becomes available.

Managing patients with pandemic influenza

Information on assessment and management of influenza patients is contained in the: