Influenza research. Close-up of flasks containing influenza virus samples. These are samples of viruses that have been recently circulating in the population. They are undergoing tests to determine their strains. There are 3 types of influenza virus, A, B and C. A and B are the most virulent. These types are further divided into strains depending on the types of surface protein that they have. The strains that are most prevalent in the population will be used in the flu vaccine for the next season. Photographed at the World Influenza Centre at the National Institute for Medical Research, Mill Hill, London, UK.
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