Apart from life and death very few biological things are absolute, and in a more practical perspective clinical medicine deals almost exclusively with likelihoods. The likelihood that a patient has a given disease is based on the presence of diagnostic factors, and differential diagnoses are always possible-if not always likely. The likelihood that a given treatment will work is established on the basis of randomised controlled trials essentially giving a chance that a treatment is better than placebo. This is reflected by the scarcity of true pathognomonic tests. Most often a disease is the result of a complicated interplay between many different endo- and exogenous pathogenic factors.