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FOOD AND DRUG ADMINISTRATION (FDA): The Public Health Service agency responsible for (among others) ensuring the safety and effectiveness of drugs, biologics, vaccines and medical devices used in the diagnosis, treatment and prevention of HIV infection, AIDS and AIDS-related opportunistic infections. The FDA also works with the blood banking industry to safeguard the nation's blood supply. See also Public Health Service.

FUNCTIONAL ANTIBODY: An antibody that binds to an antigen and has an effect. For example, neutralizing antibodies inactivate HIV or prevent it from infecting other cells. See also Antibodies; Antigen.

FUNGUS: 1. A general term used to denote a class of microbes including mushrooms, yeasts and molds. 2. Fungi, which were once classified as plants, have since been reclassified as unmoving organisms that lack chlorophyll. Mycologists (scientists working with fungi) estimate that there are 100,000 species of fungi, ranging from baker's yeast to dermatophytes (fungi that cause ringworm and athlete's foot) to potentially invasive species such as Candida albicans and Aspergillus. As many as 150 of these organisms have now been linked to animal or human diseases.