This study is being done to help determine whether patients with severe sepsis (overwhelming inflammation in the body as a result of an infection) lose muscle and become weak more rapidly than patients with other severe illnesses. Weakness and muscle loss that develops after a severe illness is a serious problem. Patients who develop weakness and have a decrease in muscle size often have to stay in the hospital longer and have a higher chance of dying. At the current time, it is not clear whether certain severe illnesses are more likely to cause weakness and muscle loss. This study will be done to measure the changes in muscle size and strength as a result of each patient's illness