Influenza (Seasonal Influenza)
Overview
Influenza is an acute respiratory infection caused by influenza viruses. The main types affecting humans are influenza A and influenza B. Type B viruses infect humans only, while type A viruses can also infect animals such as birds and pigs.
Influenza viruses change gradually over time through a process known as antigenic drift. This produces new virus strains that the immune system may not fully recognise, so people can become infected more than once.
Most people recover within about one week. However, influenza can lead to serious complications in older adults, young children, pregnant women and people with chronic illnesses.
Symptoms
- Fever and chills
- Cough
- Sore throat
- Headache
- Muscle aches
- Fatigue
- Nausea, vomiting or diarrhoea (more common in children)
Transmission
Influenza spreads mainly through respiratory droplets when infected people cough, sneeze or talk. The virus may also spread when contaminated hands touch the eyes, nose or mouth.
Incubation Period
Usually 1–4 days after infection
Infectious Period
About 1 day before symptoms appear
Usually up to about 5–7 days after symptom onset (longer in young children or immunocompromised patients)
High-Risk Groups
- Young children
- Older adults (especially aged 65 years or above)
- Pregnant women
- People with chronic diseases such as heart disease, lung disease, kidney disease, diabetes or cancer
Complications
- Pneumonia (most common)
- Bronchitis
- Sinusitis
- Myositis
- Myocarditis
- Encephalopathy
Treatment
- Rest and drink adequate fluids
- Paracetamol may help relieve fever and pain
- Children and adolescents under 18 years should avoid aspirin
- Antiviral medicines (such as oseltamivir) may shorten illness if started early
- Antibiotics are not effective against viral infections
Influenza Vaccination
Seasonal influenza vaccination is one of the most effective ways to prevent influenza and its complications. When the vaccine strain matches circulating viruses, vaccination can reduce the risk of influenza by about 40–60%.
Recommended Groups for Vaccination:
- Healthcare workers
- People aged 50 years or above
- Pregnant women
- Residents of nursing homes or disability care facilities
- People with chronic diseases
- Children aged 6 months to under 18 years
- Poultry and pig industry workers
Prevention
- Avoid close contact with people with flu-like symptoms
- Wash hands frequently
- Avoid touching eyes, nose and mouth with unwashed hands
- Cover mouth and nose when coughing or sneezing
- Stay home and rest when unwell
- Receive influenza vaccination every year
Other Types of Influenza
- Swine flu (H1N1) emerged in 2009 and resulted from genetic reassortment of swine, avian and human influenza viruses.
- Avian influenza (H5N1) mainly infects poultry. Human infection is uncommon but may occur after contact with infected birds or their droppings.
Last updated: March 2026
