Once influenza viruses enter the airways, they infect and destroy the ciliated epithelial cells lining the nose, trachea and bronchi. New virus particles leave each infected cell and invade neighbouring cells, so large areas of epithelium are damaged. This produces a sore throat, a dry cough and a runny nose.The loss of cilia means the mucociliary escalator no longer sweeps mucus and trapped microorganisms up out of the lungs. Bacteria can then settle in the lungs, so a common and dangerous complication of flu is secondary bacterial pneumonia, for example caused by Streptococcus pneumoniae. In pneumonia the alveoli fill with fluid, reducing gas exchange.Many symptoms come from the body's own immune response. Infected cells and white blood cells release chemicals called cytokines, which cause:
• a high fever, often above 38 °C, with chills
• muscle aches, headache and tiredness
• loss of appetite.Most healthy adults recover within one to two weeks. Flu is more dangerous for the elderly, babies, pregnant women and people with heart or lung disease or a weakened immune system. In severe cases the virus spreads deeper into the lungs, causing viral pneumonia, or triggers an excessive immune reaction (a cytokine storm) that damages the lungs and other organs.Worldwide, seasonal flu causes an estimated 290 000 to 650 000 deaths each year, and pandemic strains can kill far more.
• a high fever, often above 38 °C, with chills
• muscle aches, headache and tiredness
• loss of appetite.Most healthy adults recover within one to two weeks. Flu is more dangerous for the elderly, babies, pregnant women and people with heart or lung disease or a weakened immune system. In severe cases the virus spreads deeper into the lungs, causing viral pneumonia, or triggers an excessive immune reaction (a cytokine storm) that damages the lungs and other organs.Worldwide, seasonal flu causes an estimated 290 000 to 650 000 deaths each year, and pandemic strains can kill far more.