The aim of this study is to show that a preemptive treatment by ganciclovir (for positive CMV viremia) or aciclovir (for positive HSV oro-pharyngeal PCR) is able to increase the number of ventilator-free days at Day 60.

Inclusion Criteria:

• mechanical ventilation > 96 hrs and expected duration of mechanical ventilation of at least 2 days
• positive blood CMV PCR (500 IU/ml) OR positive oropharyngeal HSV PCR
• age > 18 years
• informed consent
• negative pregnancy test

Exclusion Criteria:

• < 18 years
• Receiving ganciclovir or aciclovir or another antiviral agent active against HSV/CMV
• Had received antiviral agent active against HSV/CMV during the previous month
• Hypersensitivity to aciclovir/ganciclovir
• Pregnancy
• Breast feeding
• Bone marrow failure
• Solid organ recipients
• Bone marrow recipients
• HIV positive patients
• Receiving immunosuppressive agents
• SAPS II > 75
• Withdrawing/withholding
• Neutropenia (< 500 mm3)
• Thrombocytopenia (< 25 G/L)
• ICU readmission