Based on widely recognized gaps in knowledge about how the virus operates in the body, Duesberg reached the conclusion that HIV is not virulent enough to cause AIDS. He argues that a microbe must satisfy three criteria in order to be considered responsible for disease: it must be 'biochemically active,' it must 'infect or kill more cells than the host can spare,' and 'the host must be genetically and immunologically permissive.' According to Duesberg, none of these criteria apply to HIV. Duesberg believes that AIDS is brought about by a way of life that includes sexual excess and drug abuse. AIDS researchers have responded to Duesberg on several levels, dismissing his ideas as the work of someone who grossly misrepresents or ignores much of the data and claiming that he is out of depth when it comes to biology and the complex interplay of the human immune system. Also, they contend that he is asking for absolute proof in a field where an enormous amount of circumstantial evidence is often enough to convict a microbe of being responsible for a specific disease. In addition to his ideas about the cause of AIDS, Duesberg questions the motives of those researching AIDS. He suggests that 'the AIDS establishment' is hiding something, that careers and bank accounts are on the line.