The flu is caused by two types of viruses in humans (influenza A and B). After infection, symptoms take between 1 to 3 days to show.
The flu is spread between people through droplets in the air.
Symptoms include: tiredness; fever; chills; headache; cough; sneezing and runny nose; poor appetite; and muscle aches.
Further health problems can develop in some people after infection with influenza including pneumonia, heart, blood system and liver complications, which can lead to death, especially in children and older people and those with pre-existing health conditions.