Gram stain: Differential staining procedure that allows categorization of bacteria into two groups (gram-positive and gram-negative) based on their ability to retain crystal violet when decolorized with an organic solvent such as ethanol.
harborage transmission: Disease transmission in which an infectious agent does not undergo morphologic or physiologic change during its time inside the vector.
hepatitis A (infectious hepatitis): Type of hepatitis that is transmitted by fecal-oral contamination. It affects mostly children and young adults, especially under conditions of overcrowding and poor sanitation. Caused by the hepatitis A virus.
hepatitis B (serum hepatitis): Type of hepatitis caused by the hepatitis B virus (HBV). Transmitted through body fluids.
herd immunity: Resistance of a population to spread of an infectious organism due to the immunity of a high proportion of the population.
host: Body of an organism that harbors another organism. The host provides a microenvironment that supports the growth and reproduction of the parasitic organism.
human immunodeficiency virus (HIV): Retrovirus that is associated with the onset of AIDS.
immune response: Response of the body to contact with an antigen that leads to the formation of antibodies and sensitized lymphocytes. Designed to render harmless the antigen and the pathogen producing it.
immunity: General ability of a host to resist developing a particular disease.
immunology: Science concerned with understanding the immune system and the many factors that are involved with producing both acquired and innate immunity.
index case: First disease case in an epidemic within a population.
infection: Invasion of a host by an agent, with subsequent establishment and multiplication of the agent. An infection may or may not lead to disease.
infectious agent: Living or quasi-living organism or particle that causes an infectious disease. Bacteria, viruses, fungi, protozoa, helminths, and prions are infectious agents.
infectious disease: Change from a state of health to a state in which part or all of a host's body cannot function normally because of the presence of an infectious agent or its products.
inflammation: Localized protective response to tissue injury or destruction. In an acute form, it is characterized by pain, heat, redness, and swelling in the injured area.
influenza (flu): Acute viral infection of the respiratory tract caused by one of three strains of influenza virus (A, B, and C).
intermediate host: Host that serves as a temporary but essential environment for the completion of a parasite's life cycle.
Koplik's spot: Lesion of the oral cavity caused by the measles virus.
Lyme disease: Tick-borne disease caused by the spirochete Borrelia burgdorferi.
lymphocyte: Type of white blood cell. Lymphocytes transmit chemical signals that help coordinate the immune system.
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