Influenza, commonly known as the flu, is a highly contagious viral infection that affects the respiratory system. The flu is caused by the influenza virus, which can be spread through contact with respiratory secretions, such as saliva or mucus, from an infected person. It can also be spread through contact with objects or surfaces that have been contaminated with the virus.

In tropical regions like Malaysia, influenza may occur throughout the year, with no clear seasonal trends, causing outbreaks more irregularly. Influenza A is usually detected more frequently than influenza B, although year-to-year variation may be considerable.

Flu can cause fever and chills, sore throat, muscle aches, fatigue, cough, headache, and runny or stuffy nose. Some people may have vomiting and diarrhea, though this is more common in children than adults. Anyone can get the flu, but it is more dangerous for some people. Infants and young children, people 65 years and older, pregnant people, and people with certain health conditions or a weakened immune system are at greatest risk of flu complications.