Flu is a very common, global disease. Each year there is a period of time (a flu season), during which most of the flu cases happen, usually during the winter, but this can vary depending on where you live. For example, in the tropics, the flu season tends to reach its highest point in the rainy season. Influenza viruses also change (or mutate) from 1 year to the next, so that your immune system does not recognize them anymore, and is less able to protect you against infection and disease. This means that, every year, scientists need to check which flu viruses are infecting people and design new vaccines to protect against these current virus strains . Each year, flu vaccines are usually made up of a mixture 3 or 4 strains that match the main circulating strains.
Sometimes, new strains of virus emerge that are completely different from the circulating virus strains. Scientists are really worried about the threat the new influenza viruses could pose to human health. Some new flu virus strains could affect more people than seasonal flu and cause a global outbreak. We call such strains pandemic strains.

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