Seasonal flu (also known as influenza) is a highly infectious illness caused by a flu virus.

The virus infects your lungs and upper airways, causing a sudden high temperature and general aches and pains.

You could also lose your appetite, feel nauseous and have a dry cough (see Symptoms). You may need to stay in bed until your symptoms get better.

Symptoms can last for up to a week.

How it is spread

The flu virus is spread in the small droplets of saliva coughed or sneezed into the air by an infected person. If you breathe in these droplets, you may become infected.

Flu can also spread if someone with the virus touches common surfaces such as door handles with unwashed hands.

Typically several different strains of flu virus circulate at the same time. In 2010-11 one of the season’s strains was H1N1, responsible for swine flu.

The infectious period

Symptoms develop one to four days (two days on average) after being infected.

People with flu are usually infectious (can spread the virus) a day before symptoms start, and remain infectious for five or six days. Children and people with weaker immune systems (such as cancer patients) may remain infectious for slightly longer.

Try to avoid all unnecessary contact with others during this infectious period.

How common is it?

Seasonal flu is a very common illness that occurs every year, usually during the winter months (October to April in Ireland).

The number of people who consult their GP with flu-like symptoms varies from year to year, but is usually between 50 and 200 for every 100,000 people. This is in addition to the many people with flu who do not see their GP.

Outlook

Your symptoms will usually peak after two to three days. You should begin to feel much better within five to eight days.

However, elderly people or those with certain medical conditions may develop a complication such as a chest infection. This can lead to serious illness and can be life-threatening.

A seasonal flu vaccination is available free for pregnant women,everyone aged 65 and older, anyone over 6 months who has a long-term illness ,and health care staff and carers.

Immune system

The immune system is the body’s defence system, which helps protect it from disease, bacteria and viruses.

Epidemic

An epidemic is a sudden outbreak of disease that spreads through a population in a short amount of time.