Centers for Disease Control. Responsible for prevention and control of diseases and response to public health emergencies. A good place to start is page on "Diseases and Conditions."
CDC Wonder Access to a wide variety of public health reports and data systems. See especially the section on "Communicable Diseases" under "Topics"
Clinical Trials.Gov A registry of federally and privately supported clincal trials in the U.S. and around the world
Ecology of Infectious Diseases Infectious diseases are spreading in animals and humans. Is our interaction with the environment somehow responsible? From the National Science Foundation.
Emerging Infectious Diseases. Monthly journal on infectious diseases whose incidence has increased in the past two decades or threatens to increase in the near future. From the Centers for Disease Control and Prevention.
FedStats Offers statistics on all topics gathered from more than 100 federal agencies.
Food and Drug Administration. Regulates safety and quality of food, human and animal drugs, biological products, and medical devices. Responsible for approval of new drugs.
Medline Plus. Online health information for the general public from the National Library of Medicine.
Morbidity and Mortality Weekly Report. Statistics on diseases reported to federal authorities by state health departments and related health information from the Centers for Disease Control and Prevention.
National Institutes of Health. The principal biomedical research agency of the federal government
National Institute of Allergy and Infectious Diseases. Sponsors research to understand, treat and prevent a variety of infectious, allergic, and immunologic diseases.
National Institutes of Health Reporter Searchable database of federally funded biomedical research projects at hospitals, universities and research centers. Formerly called CRISP: Computer Retrieval of Information on Scientific Projects.
Science.Gov Searches more than 50 databases and over 2100 science-related websites from 13 federal agencies.
European Centre for Disease Prevention and Control. Speeches, publications, research projects and legal documents from the agency responsible for managing and controlling communicable diseases within Europe.
UNAIDS. Full name: Joint United Nations Programme on HIV/AIDS. Brings together the efforts and resources of ten United Nations system organizations to prevent new HIV infections, care for people living with HIV, and lessen the impact of the epidemic.
World Health Organization. The coordinating authority for health matters within the United Nations. Provides leadership on global health matters, shapes health research agenda, monitors and assesses health trends, and develops policy options.