The purpose of this study is to gain knowledge that will be used to design further interventional studies to better define beneficial therapies related to gender, infection, and critical illness, such as modulation of hormone levels in a sex-specific manner. This is a two-year observational study in which approximately a total of 570 patients will be enrolled. All patients will be 18 years of age or older and admitted to the Intensive Care Unit (ICU) for at least 48 hours. Patients will be studied while receiving accepted and approved therapy according to local standards. Data obtained will be that which would normally be considered part of a standard complete medical history. Specimens will come from patients either as wasted blood (University of Virginia) or blood drawn from patients specifically for this purpose (Vanderbilt University). The maximum blood removed will be 30 ml twice weekly and is considered a minor risk. All specimens analyzed, regardless of institution, are done so in a blinded manner, identified only by study number and specimen number or through password and encryption protected servers when communicated electronically. Since the cohort is comprised of all patients admitted to an ICU, recruitment in the normal sense is not practical. An initial comparison of demographic data, severity of illness, frequency of comorbidities, hormone and cytokine levels, and outcome variables will be compared between males and females treated for infection. All patients will be followed daily until death or discharge from the ICU. It is anticipated that a similar risk of infection for men and women overall will be determined. Specific Aim I of the study is to prospectively determine and compare the incidence of and associated mortality from hospital-acquired infections in a large, critically-ill population of pre-menopausal women, post-menopausal women, and men after controlling for multiple pre-defined confounding variables. The Specific Aim II of the study is to determine the relationship between sex hormonal status, systemic cytokine levels, and the incidence of and outcome from hospital-acquired infections, as well as the relative contribution of infection and end-of-life decisions to outcome in subjects dying under study.