Among vulnerable populations during an influenza pandemic are persons with or at risk for HIV infection, tuberculosis, or chronic viral hepatitis. HIV-infected persons have higher rates of hospitalization, prolonged illness, and increased mortality from influenza compared with the general population. Persons with tuberculosis and chronic viral hepatitis may also be at increased risk of morbidity and mortality from influenza because of altered immunity and chronic illness. These populations also face social and structural barriers that will be exacerbated by a pandemic. Existing infrastructure should be expanded and pandemic planning should include preparations to reduce the risks for these populations.