The delay in deciding that the chytrid was pathogenic was due to its superficial location in frogs, lack of host response to its presence, and that no other agent in such a superficial position has killed so effectively. Evidence of the chytrids pathogenicity was demonstrated by infection experiments in 1997 by Lee Berger (Australia) and in 1998 by Don Nichols (Washington). Lee's first experiments did not fulfil Koch's postulates since she was unable to grow the fungus and used infected frog skin to reproduce the disease (Berger et al 1998). Don fulfilled Koch's postulates by using a pure strain to experimentally infect frogs and by reisolating the chytrid from frogs that died of chytridiomycosis.