Other research by Tumpey and colleagues who reconstructed the H1N1 virus of 1918 came to the conclusion that it was most notably the polymerase genes and the HA and NA genes that caused the extreme virulence of this virus.[12] On 18 January 2007, Kobasa et al. reported that infected monkeys (Macaca fascicularis) exhibited classic symptoms of the 1918 pandemic and died from a cytokine storm.[20]