1.1 What is influenza or flu?
Flu is an illness resulting from infection by an influenza virus. It is highly infectious and can spread easily from person to person. Because the flu virus constantly changes there are many different strains of flu. Some are more infectious and cause more severe illness than others.
1.1.1 Symptoms
People are affected by flu with varying degrees of severity ranging from minor symptoms through to pneumonia and death.
Symptoms are generally of sudden onset and include:
- headache
- severe weakness and fatigue
- fever
- cough
- aching muscles and joints
- sore throat
- runny nose.
The symptoms of pandemic flu are similar to ordinary, seasonal flu. However, in the case of pandemic flu, these symptoms are likely to be worse, resulting in more severe illness and possibly death.
1.1.2 How flu spreads
Influenza viruses are easily passed from person to person through the air. You can catch flu through breathing in air containing droplets or aerosols containing the virus produced when an infected person talks, coughs or sneezes. You can also catch it through touching an infected person or surface contaminated with the virus, and then touching your face.
Flu viruses have an incubation period - the time a person is infected with flu before showing symptoms - of one to three days People are likely to be infectious from just before symptoms develop until four to five days after the onset of symptoms. Children, however, tend to be infectious for much longer.
1.1.3 Different types of flu virus
Flu viruses are divided into three main groups: influenza A, B and C. Type A viruses are the source of most 'ordinary' flu epidemics and have caused all previous pandemics. Whereas influenza B and C viruses infect humans only, influenza A viruses also infect birds and other animals such pigs and horses. This unique ability to jump the species barrier enables influenza A viruses to cause pandemics.
Table 1.1 Influenza virus types
| Influenza types | Hosts |
|---|
| Type A | Humans, birds, pigs and horses |
|---|
| Type B | Humans only |
|---|
| Type C | Humans only |
|---|
