The immune system is the part of the body that protects against infection. The immune system often doesn't work as effectively as people get older. This research is being done to find out how the immune systems in older people who are over age 75 respond to influenza vaccine (flu shot). We also want to find out if chronic cytomegalovirus (CMV) infection, a common virus infection in older persons affects the immune response in people older than 75 years of age who receive a flu shot. The Flu Shot is a vaccine approved for the prevention of influenza (Flu) infections and is recommended every year for all persons 50 years and older. People who are older than 75 years of age are considered healthy or frail may join. A total of 1025 persons will be participating in this study. In order to determine if you are qualified for the study, we would ask you to answer a few questions over the phone that will take approximately 5 minutes. If you qualify and agree to proceed, you will be asked to come to Johns Hopkins Bayview Medical Center or, if you are unable to come to Bayview, one of our staff can visit you at your home. During that visit we obtain consent, review your medical history, and measure your vital signs, walking speed and grip strength. We will also administer a few brief questionnaires and collect urine and blood samples. We will then give you the Influenza vaccine for free. 7 days post receiving your Influenza Vaccine we will collect a small blood sample for further immune system testing. also, you will be asked to complete a 12-question survey which will assess your symptoms over the past 7 days (post receiving the Influenza vaccine). Also 4 weeks post receiving your Influenza vaccine you will be asked to complete a third visit that will include follow up health questionnaires and an influenza symptoms assessment questionnaire and vital signs. A third blood draw will be collected (approximately 10 teaspoons) to measure immune responses to the influenza vaccine. In addition, you will receive your test result (CBC/w/Diff.) from visit #1. Throughout the influenza study season, we will call you once a week to ask about your general health and any Flu-like symptoms. These calls will be made throughout the Flu season which typically lasts through May. If you begin to have any influenza like symptoms at any time during the study, we ask that you call our office to report these symptoms so that we may perform vital signs, nasal swab to confirm influenza, and a fourth blood draw to look at the immune response and protection of influenza vaccine.