Several limitations of our study deserve to be acknowledged. We found considerable heterogeneity in the methodology used to assess CMV infection and in patient characteristics. As noted in the Results, the frequency and type of CMV monitoring influenced the rate of CMV infection. Although all the studies in this review were conducted in the ICU, the overall mortality of the studied patients ranged from 5 to 71% [7, 15] , suggesting that study populations were significantly diverse. Furthermore, these studies were published over a period spanning nearly two decades in different regions with diverse clinical practices. Considering the heterogeneity of available data, quantitative analyses such as meta-analysis can be misleading [66, 67] and our results are therefore presented in a descriptive fashion only. Second, while we excluded the recipients of iatrogenic immunosuppressive agents that enhance the risk of CMV reactivation, critically ill patients in whom corticosteroids were employed were included. Controversy abounds whether corticosteroids alone without other immunosuppressive agents lead to reactivation of CMV from latency or merely promote the replication of activated virus [68] [69] [70] .