Prevalence of Replication of Human Herpes Virus 6 (HHV6) in Blood and Skin During Exanthemia, in Patients With Hemopathy. Is There a Correlation With the Etiology of the Exanthema?

To look for the replication of HHV6 by PCR in the skin, in these patients with haemopathies presenting an exanthema. This will make it possible to evaluate the prevalence of the positivity of this PCR in the skin. Then the investigator can investigate whether there is an association between the positivity of HHV6 PCR in the blood and / or skin and the viral etiology of the exanthema. If this association exists, it will make it possible to improve the diagnosis in the context of the exanthema and thus to improve the therapeutic management of these patients.