Coloured composite transmission electron micrograph (TEM) of H5N1 avian influenza virus particles (bird flu). This highly pathogenic RNA (ribonucleic acid) virus spreads rapidly among birds, but in rare cases the H5N1 strain has spread to humans through close contact with an infected bird. Although H5N1 doesn't infect people easily and isn't usually spread from human to human, several people have been infected around the world, leading to a number of deaths. Symptoms of bird flu include high fever, headache, muscle pain, shortness of breath, nausea, diarrhoea and sickness. The virus was first identified in 1996 in southern China and Hong Kong and was traced to a poultry farm.