4. Free of obvious health problems or chronic illnesses (i.e., recent exacerbation or acute episode of chronic illness in the last 3 months) as established by medical history, review of systems and clinical examination before entering the study. This includes any mental condition that would interfere with subject self-assessment. Subjects with pre-existing stable disease, defined as no significant disease diagnosed in the month prior to study vaccine receipt and disease not requiring significant change in therapy or hospitalization for worsening disease 3 months before receipt of study vaccine are eligible.