Yearly influenza (flu) epidemics cause a large burden of lost work and school time among younger persons and excess hospitalizations and deaths among the elderly. Injectable flu vaccines are generally believed to be effective, but formal studies using modern culture methods to prove this are limited. This study will compare the rate of respiratory illnesses associated with positive influenza virus cultures in healthy younger adults (a group at low risk for serious influenza complications) who have received Fluviral (an injectable flu vaccine) with the rate of such illnesses in healthy younger adults who have received an injection of saline (salt water). The rate of local vaccine reactions and other symptoms will be compared between the vaccine and salt water groups, and the immune responses to the vaccine measured.