Seasonal flu is another name for the flu virus that occurs at a time when outbreaks are common. In the UK this is often throughout the colder winter months.
The strain of the seasonal flu virus is usually different to those seen the previous seasons due to the virus changing or mutating as its passed from person to person.
Treatment of the seasonal flu can be much more difficult using flu vaccines due to the changes in the virus caused my the mutation. As a results the flu vaccines used from year to year also change.
This mutated flu virus means that those in high risk groups such as the young, old or ill are especially vulnerable to the seasonal flu virus. This is because their immune system is often not strong enough to fight the infection and any resistance built up to the previous seasonal flu strains are no longer able to fully right it.