BIOL0131_L4 handout_2009

BIOL0131_L4 handout_2009 - A refresher on denitions A...

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1 Basic Medical Microbiology Lecture 4 Host Parasite Interactions A refresher on deFnitions A pathogen is a biological agent that causes disease The host is the human individual colonized by the pathogen Most human microbes are commensal – Mutual beneFt Some transient commensal microbes may become opportunistic pathogens – Causing disease when conditions such as reduced immune system function allow Primary pathogens cause disease in otherwise healthy individuals – E.g. In±uenza virus, typhoid Bacillus etc The term virulence is used to express degrees of pathogenicity ²ACT • Infectious disease caused by microbes is the leading cause of death in humans worldwide • As a species, most Homo sapiens individuals simply do not live long enough to die of old age! The emergence of infectious disease • Many well-known infectious diseases are fairly ‘recent’ to humanity – E.g. bubonic plague (Black Death) caused by the bacterium Yersinia pestis , which killed a third to a half of all humans was only 700 years ago (14th Century) – E.g. The Spanish ²lu pandemic of 1918 caused by In±uenza A virus strain H1N1 killed up to 100 million people worldwide, more than the plague! – Probably less common in ancient societies – The effects on the human gene pool are largely unknown • Those that dies were likely just as important genetically as those that survived
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2 Yersinia pestis Influenza virus • Today, disease is more prevalent in areas of overcrowding, poverty, civil unrest, famine and war • Outgrowth of human populations into wild areas has probably contributed to some diseases emerging – E.g. Legionnaires disease caused by Legionella sp. Fnding a new host in human macrophages rather than their normal aquatic protozoan host, and being spread through aerosols in showers, supermarket chillers and also air-con ducting – Others include Lymes disease and HIV • Also human development of technology – E.g. food processing led to the spread of E.coli O157:H7 which causes food poisoning • But in the history of mankind the most important impact on the evolution of human disease was the
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BIOL0131_L4 handout_2009 - A refresher on denitions A...

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