Infection and Immunity Principles of Infectious Disease PDF

# INFECTION AND IMMUNITY ## Principles of Infectious Disease - **Host** - organism which provides nutrients, etc. to another organism - **Parasite** - organism which lives at the expense of (and may even harm) its host; the parasite is generally smaller than the host and is metabolically dependent u...

# INFECTION AND IMMUNITY ## Principles of Infectious Disease - **Host** - organism which provides nutrients, etc. to another organism - **Parasite** - organism which lives at the expense of (and may even harm) its host; the parasite is generally smaller than the host and is metabolically dependent upon it - **Disease** - an upset in the homeostasis of the host, resulting in generation of observable changes - **Symptom** - subjective evidence of damage to the host (headache, anorexia) - **Sign** - objective evidence of damage to the host (fever, rash, vomiting) - **Infectious disease** - one in which detrimental changes in health of the host occur as a result of damage caused by a parasite - **Pathogen** - microorganism that is capable of causing disease - **Virulence** - a measure of pathogenicity, which is the ability to cause disease - **Virulent** - microorganisms that readily cause disease (only small numbers of the microorganism are required to initiate and sustain infection) - **Opportunistic** - microorganisms that may or may not cause disease generally colonize but do not infect the host when usually found associated with a host, called normal microbiota, can cause disease if they are inadvertently introduced intro a site where they do not usually reside, especially inside host tissues. - **Avirulent** - microorganisms that do not cause disease - **Attenuated** - microorganism with reduced ability to cause disease. ## Koch's Postulates If a microorganism is the causative (etiologic) agent of an infectious disease, it must be: - Present in every case of the disease, but absent from the healthy host - Isolated and grown in pure culture - Able to cause the disease when a pure culture is inoculated into a healthy host - Re-isolated from the host that was inoculated with the pure culture. ## Steps in Pathogenesis To cause disease, a pathogen must - - Contact the host - be transmissible - Colonize the host - adhere to and grow or multiple on host surfaces - Infect the host- proliferate in host cells or tissues - Evade the host defense system - by avoiding contact that will damage it. - Damage host tissues - by physical (mechanical) or chemical means. ## Virulence Factors Factors responsible for the virulence of a microorganism because they influence its ability to cause disease by affecting its invasiveness and/or its toxigenicity. - **Adhesins** - promote attachment to host cells and tissues. - pili - Escherichia coli and Neisseria gonorrhoeae use these to attach to urethral cells; Salmonella and Escherichia coli uses them to attach to intestinal cells. ## Definition of Antigen and Antibody - An **antigen** is any substance that is capable of inducing an immune response when introduced into an organism - An **antibody** is a protein (an immune globulin molecule) produced by an organism in response to stimulation by an antigen.