It peaks at different times in different winters. It is not possible to predict when it will peak, the size of the peak, which age groups will be most affected, or which strains will be the commonest. Influenza viruses infect the respiratory tract, and causes symptoms including a cold, cough and sore throat but also systemic symptoms of fever, headache, runny sore eyes and aching muscles. Some other viruses cause these symptoms, and laboratory tests are required to determine the cause. Patients who have systemic symptoms as well as a cold and a cough are, therefore, described as having flu or flu-like illness.
Most people recover within a few days, but some develop complications such as pneumonia.
It is estimated about 2000 to 3000 people die each year as a result of flu. There are many different strains within each type of influenza virus. Every year there are minor changes in the strains that are found in the population. Some years, new strains appear, and some of these new strains may spread widely and cause epidemics in many countries. The World Health Organization monitors data on influenza viruses from laboratories in different countries, and makes recommendations about which strains should be included in vaccines.