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.
How yellow fever is spread. The virus that causes yellow fever is passed to humans through the bites of infected mosquitoes. The mosquitoes that spread the infection are usually active and bite during daylight hours, from dusk until dawn, and are found in both urban and rural areas.