bacteria . These cases probably reflect only a small percentage of the actual number of bacterial infections occurring in the general population , and usually represent the most severe cases . In developing countries , a variety of bacterial infections often exert a devastating effect on the health of the inhabitants . Malnutrition , parasitic infections , and poor sanitation are a few of the factors contributing to the increased susceptibility of these individuals to bacterial pathogens . The World Health Organization has estimated that each year 3 million people die of tuberculosis , 500,000 die of pertussis , and 25,000 die of typhoid . Diarrheal diseases , many of which are bacterial , are the second leading cause of death in the world ( after cardiovascular diseases ), killing 5 million people annually .
Many bacterial diseases can be viewed as a failure of the bacterium to adapt , since a well-adapted parasite ideally thrives in its host without causing significant damage . Relatively nonvirulent ( i . e ., well-adapted ) microorganisms can cause disease under special conditions ; for example , if they are present in unusually large numbers , if the host ’ s defenses are impaired , ( e . g ., AIDS and chemotherapy ) or if anaerobic conditions exist . Pathogenic bacteria constitute only a small proportion of bacterial species ; many non-pathogenic bacteria are beneficial to humans ( i . e ., intestinal flora produce vitamin K ) and participate in essential processes such as nitrogen fixation , waste breakdown , food production , drug preparation , and environmental bio-remediation .
In recent years , medical scientists have concentrated on the study of pathogenic mechanisms and host defenses . Understanding host-parasite relationships involving specific pathogens requires familiarity with the fundamental characteristics of the bacterium , the host , and their interactions . Therefore , this section first presents with the basic concepts of the immune response , bacterial structure , taxonomy , metabolism , and genetics . Subsequent chapters emphasize normal relationships among bacteria on external surfaces ; mechanisms by which microorganisms damage the host ; host defense mechanisms ; source and distribution of pathogens ( epidemiology ); principles of diagnosis ; and mechanisms of action of antimicrobial drugs . These chapters provide the basis for the next chapters devoted to specific bacterial pathogens and the diseases they cause . The bacteria in these chapters are grouped on the basis of physical , chemical , and biologic characteristics . These similarities do not necessarily indicate that their diseases are similar ; widely divergent diseases may be caused by bacteria in the same group .
References :
Baron S , editor . Medical Microbiology . 4th edition . Galveston ( TX ): University of Texas Medical Branch at Galveston ; 1996 . Introduction to Bacteriology . Available from : https :// www . ncbi . nlm . nih . gov / books / NBK8120 /
Pearce , P . Public Health Bacteriology Laboratory Manual . Wichita Falls / Wichita County Health Department . Wichita Falls , TX . 1970 .
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