Types. Economists Ronald G. Ehrenberg and Robert Smith, authors of Modern Labor Economics, identify two main types of labor unions: craft unions and industrial unions. Craft unions represent workers in a specific occupation.lthough union membership in the United States peaked in the mid-20th century and has declined since then, various types of labor unions remain an influential force in American business and politics.
U.S. Army Physicians Discovered the Cause of Yellow Fever. August 27, 1900. No one knew what caused the often-deadly yellow fever, but it occurred in epidemic proportions, with one person after another in a given area becoming sick. People feared the mysterious disease, until U.S. Army physician James Carroll endangered his own health in the name of science.
This is the bacterium that causes cholera. The germ theory of disease states that some diseases are caused by microorganisms. These small organisms, too small to see without magnification, invade humans, animals, and other living hosts. Their growth and reproduction within their hosts can cause a disease.he germ theory was proposed in the mid-16th century and gained widespread credence when substantiated by scientific discoveries of the 17th through the late 19th century. It supplanted earlier explanations for disease, such as Galen's miasma theory.