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Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interests include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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The Impact of Microbes on the Environment and Human Activities (page 3)

(This chapter has 4 pages)

© Kenneth Todar, PhD

Harmful Effects of Microbes

The primary harmful effects of microbes upon our existence and civilization is that they are an important cause of disease in animals and crop plants, and they are agents of spoilage and decomposition of our foods, textiles and dwellings.

Microbes Cause Infectious Disease

A microbe which is capable of causing infectious disease in an animal or plant is called a pathogen. Four groups of microbes contain pathogens: bacteria, fungi, protozoa and the viruses. Only the archaea and algae are lacking pathogens. Pathogens are the cause of infectious diseases.


(L) Borrelia burgdorferi, the spirochete that causes Lyme Disease. (R) Influenza virus, cause of flu. CDC.

Historically, infectious diseases are the most significant cause of death in humans. Until the beginning of the Twentieth Century, it is estimated that more than half the people who ever lived died from smallpox, caused by a virus, or malaria, caused by a protozoan. Bacteria, too, have been the cause of some of the most deadly diseases and widespread epidemics of human civilization. Bacterial diseases such as tuberculosis, typhus, plague, diphtheria, typhoid fever, cholera, dysentery and pneumonia have taken a huge toll on humanity.

Deaths from infectious diseases declined markedly in the United States during the 20th century. This contributed to the nearly 30-year increase in life expectancy during this period. In 1900, the three leading causes of death were pneumonia, tuberculosis (TB), and diarrhea and enteritis, which (together with diphtheria) caused one third of all deaths. In 1997, heart disease and cancers accounted for 55% of all deaths, with 4.5% attributable to pneumonia, influenza, and human immunodeficiency virus (HIV) infection . However, one of the most devastating epidemics in human history occurred during the 20th century: the 1918 influenza pandemic that resulted in 20 million deaths, including 500,000 in the United States in less than 1 year - more than have died in as short a time during any war or famine in the world.

HIV infection, first recognized in 1981, has caused a pandemic that is still in progress, affecting 33 million people and causing an estimated 13.9 million deaths. This illustrates the volatility of infectious disease and the unpredictability of disease emergence and points us to the challenges ahead.

Progress in the 20th century is based on the 19th century discovery of microorganisms as the cause of many serious diseases (e.g., cholera and TB). Disease control resulted from improvements in sanitation and hygiene, the discovery of antibiotics, the implementation of universal childhood vaccination programs, and technological advances in detecting and monitoring infectious disease. 


Incidence of infectious disease as a cause of mortality 1900 to 1996. CDC.


Water purification, immunization (vaccination), and modern antibiotic therapy (all developments in the field of bacteriology) have dramatically reduced the morbidity and the mortality of infectious disease during the Twentieth Century, at least in the developed world where these are acceptable cultural practices. However, many new microbial pathogens have been recognized in the past 30 years  and many "old" bacterial pathogens, such as Staphylococcus aureus and Mycobacterium tuberculosis, have emerged with new forms of virulence and new patterns of resistance to antimicrobial agents.

Microbes are also the cause of many diseases in plants, which, if crop plants or forest resources, may have important economic or social consequences.





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Kenneth Todar is an emeritus lecturer at University of Wisconsin-Madison. He has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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