INFLUENZA - Frequently asked questions

What is influenza?
Influenza, commonly called "flu", is an acute respiratory illness that affects the upper and/or lower parts of the respiratory tract and is caused by an influenza virus, usually of type A or B. Patients become ill between 18 and 72 hours after being infected. The most common symptoms of uncomplicated influenza are an abrupt onset of fever, shivering, headache, muscle ache and a dry cough. Cough is often severe and protracted, but other manifestations are usually self-limited with recovery in 2-7 days.

How serious is influenza?
For most people, influenza virus infection is just a nasty experience, but in some instances it can lead to illnesses that are more serious. The most common complications of influenza are bronchitis and secondary bacterial pneumonia. These illnesses may require treatment in a hospital and can be life-threatening especially in "high-risk" people e.g. the elderly and people suffering from chronic heart disease. During the 1918 pandemic (see below), though, influenza also caused serious illness in young healthy individuals.

How is influenza diagnosed?
In general, physicians base their diagnosis on clinical symptoms. It may be difficult to differentiate influenza from other acute respiratory infections. During an epidemic, about 60-70% of patients with a clear influenza-like illness actually have influenza; the remainder may suffer from other viral infections. Laboratory tests allow the detection of the influenza virus or viral protein in throat and/or nose swabs and antibodies in blood.

What distinguishes influenza from the common cold?
Many people confuse influenza with a heavy cold. However, influenza is usually a more severe illness. The symptoms of a common cold tend to be shorter and complications like pneumonia are experienced rarely.

What are the symptoms of the common cold?
Cold symptoms are a runny nose, sneezing, watery eyes and throat irritation. These symptoms are limited to the upper respiratory tract and do not usually include a fever or body aches.

Are there other viruses that can cause similar illnesses?
Besides influenza viruses, other respiratory infections may cause an influenza-like illness. Infections with a rhinovirus, respiratory syncytial virus (RSV), adenovirus and parainfluenza viruses may induce a disease that can be mistaken for influenza as they are frequently associated with similar symptoms.

Can influenza be prevented?
The only proven effective method of prevention is vaccination. Antiviral drugs may also prevent influenza, but they are not registered for prophylaxis in Europe (see below).

Why do people usually get influenza during the winter?
In temperate climates, influenza generally affects people from November to March. It can occur all year round in tropical climates. A possible explanation for the high level of influenza activity during the winter is that the congregation of people indoors facilitates the transmission of the virus and viruses survive longer due to the dry indoor air.

How is influenza spread?
The virus is efficiently transmitted between individuals through droplets of aerosols generated by coughs and sneezes. The virus in the aerosols subsequently infects cells in the lining of the breathing passages and viral replication occurs within four to six hours.

How long are persons infectious?
The infectious period of an influenza patient is 3-5 days from the onset of symptoms in adults and up to 7 days in young children.

What is the treatment?
As with most viral illnesses, influenza needs to run its course. Those suffering from influenza can use a painkiller such as paracetamol to help relieve headache and muscle ache and to reduce their temperature. In some countries the antiviral agents amantadine and rimantadine are available to treat influenza A infections: both are equally effective but amantadine can cause neurological and rimantadine gastrointestinal adverse effects. In addition, drug resistance to both of these agents appears to arise frequently.
Neuraminidase inhibitors are a new class of medicines for influenza; they are aimed at treating the underlying cause of the illness (i.e. influenza virus replication), not just the symptoms. These antiviral drugs may decrease the length of time that symptoms persist by 1 to 2.5 days if started within 48 hours of onset. At present, zanamivir (Relenza®) and oseltamivir (Tamiflu®) are registered for influenza treatment in Europe. Oseltamivir (Tamiflu®) can also be used for prophylaxis (e.g. if influenza gets a foothold in a closed community like a nursing home or hospital ward, it can be used to protect the residents or patients from succumbing to the infection).

How many people suffer from influenza?
Influenza epidemics occur virtually every year, although the extent and severity of such epidemics vary widely. Sometimes pandemics occur (see below). The most frequent, extensive and severe outbreaks are caused by the influenza A virus. Illness and death from influenza outbreaks continue to be substantial. In the interpandemic period, death occurs primarily in individuals with underlying diseases who have been characterized as being at "high risk" for complications of influenza. It is estimated that approximately 10-15% of people get influenza around the world each year. During major epidemics, the attack rate of influenza may be as high as 50%.

What is an epidemic?
The occurrence in a community or region of cases of influenza clearly in excess of normal year-round frequency.

What is a pandemic?
In general, a pandemic is an epidemic occurring over a very wide area (at least a whole continent) and usually affecting a large portion of the population. According to this definition, there is an influenza pandemic each year. With influenza, however, the term pandemic is restricted to a worldwide epidemic caused by a new influenza virus subtype. Such a new subtype is characterized by major changes in protein antigens found on the surface of the influenza virus particle. As most people have not had the chance to become immune to these new strains, widespread infection may occur. Pandemics only occur with the influenza A virus.

When is the next pandemic likely to occur?
It is not known. In the 20th century, severe pandemics occurred in 1918 ("Spanish Flu"), 1957 ("Asian Flu") and 1968 ("Hong Kong Flu").

Where is the new virus likely to come from?
Past experience would suggest it will first appear in Asia, but it could come from anywhere.

Will the next pandemic occur during the winter?
Not necessarily. It could occur at any time of the year. The famous "Spanish Flu" started in March-July 1918. Still, the main waves of mortality occurred during the winter of 1918.

What is avian influenza?
Avian influenza, or "bird flu", is a contagious disease caused by influenza viruses that normally infect only birds and, less commonly, pigs. Domestic poultry is especially vulnerable to infections. It can cause large outbreaks associated with high mortality. In these instances, the term “highly pathogenic avian influenza” (HPAI) is used. Outbreaks in poultry may spread rapidly. For more information click here.


Last updated: 21 November 2005