Cytomegalovirus (CMV) is a latent infection virus that is widespread in the population. CMV may reactivate under certain circumstances, and its hazardous nature has been proven, especially in immunosuppressed patients. However in recent years, some studies have found that CMV reactivation also exists in apparently immunocompetent patients during their critical illness. In these non-immunosuppressed patients, the presence of sepsis and its immunomodulatory effects may lead to reactivation of dormant viral infections. Studies found the incidence of CMV reactivation in up to one-fourth to half of these non-immunosuppressed patients during their critical illness. These studies also documented significantly higher length of ICU stay and mortality in critically ill patients with CMV reactivation. CMV reactivation has also been studied in specific critically ill cohorts, exhibiting there impact on mortality. David S Y Yong et al studied the effect of CMV reactivation on mortality in immunocompetent acute respiratory distress syndrome (ARDS) patients. In a recent study, it was found that among cohort of non-immunosuppressed patients with acute respiratory distress syndrome (ARDS), CMV reactivation occurred in 27 % of patients which was associated with overall increased ICU mortality. In another study among septic shock cohort (329 patients), herpesvirus reactivations were documented in 68% patients without prior immunodeficiency and concluded that reactivations could be independently associated with mortality. However currently, no study is available investigating the CMV reactivation and it's kinetics during critical illness in non immunocompetent patients requiring prolonged mechanical ventilation. In this planned observational study, we aim to find out the prevalence of Cytomegalovirus (CMV) reactivation, viral load and it's association with the severity of illness in non immunosuppressed ICU patients requiring prolong mechanical ventilation.