The polymerase chain reaction (PCR) has revolutionized diagnosis during these last years. It allows the detection of bacteria, viruses and parasites, dead or alive, in various biological tissues. It has given access to diagnoses in formerly undiagnosed infectious diseases. The enthusiasm which followed the use of this technique made it possible to hope that many unexplained diseases, such as multiple sclerosis [1], sarcoidosis [2] and Crohn's disease [3] would benefit from this novel method. Unfortunately, the majority of the results published and obtained by PCR in these diseases have proved to be false-positives [4]. A paper on the refutation of Rickettsia helvetica as a cause of sarcoidosis is published in this issue [5].
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