Until the coming of the Europeans, the New World was free of smallpox, typhus, cholera, and measles--the focus of this article. When Cortez came to invade Mexico, he had with him a silent ally more potent than his small Spanish army. That insidious ally was infectious disease, to which Aztecs and other Native Americans had no immunity.

When he finally entered Tenochtitlan (Mexico City today) in 1520, the year after he first arrived in the New World, he found half of the inhabitants infected with smallpox. In just the first epidemic, nearly 50% had died. Eleven years later, a second epidemic devastated Mexico, and this too was introduced from Spanish ships. By 1595, over 18 million people had died of smallpox, mumps, measles and other European diseases. (For a further narrative, see Cartwright amongst the resources below.)



This viral disease is relatively recent in human history and is believed to have come from cattle or possibly monkeys. It probably originated in the area around the Caspian Sea. It became epidemic throughout Europe following the Crusades, and was brought to the New World several times by explorers. Epidemic after epidemic swept Europe following the Crusades during the Middle Ages. It killed one third of the people and producing permanent pock marks in another third of its victims.

Smallpox is a classic epidemic disease that was sustainable only by large human populations.


Cowpox, a benign related disease in cows produces few symptoms in humans and confers immunity to smallpox. This is an observation that led to successful mass immunization against the disease. (My note: This illustrates cross immunity where immunity to one disease works against a second, closely related one.)

(Medical trivia: The vaccine BCG for tuberculosis has shown to confer little immunity to TB, but strangely provides immunity to leprosy about 50% of the time. BCG is not used in the United States. See Wills, p 249.)

(My historical note: Smallpox no longer ravages humanity. Until twenty years ago, vaccination with cowpox virus was used--a technique initiated by Jenner in 1796. Ancient peoples in India, China, and Persia observed permanent immunity following recovery from smallpox and reasoned that innoculation with the dried crusts of pustules. Most worked, but occasionally some persons died from a full blown case of smallpox.)

The deadly smallpox virus, which disfigures and often causes blindness and death, was eradicated from nature early in the 1980s. A major triumph for public health, smallpox eradication remains a landmark in disease management. The last outbreak was in Somalia in 1977.

The virus survives today in freezers located in Atlanta (Center for Disease Control) and Koltsovo, Russia (near Moscow). Those laboratory specimens were due for incineration in June, 1995, but this was postponed by the World Health Organization in the interest of research.

(My note: The fate of smallpox specimens in Atlanta and Moscow remains uncertain. President Clinton has decided to keep the US stock because of fears of bio terrorism [Chicago Tribune April 23, 1999]. The World Health Organization has decided to delay the destruction of the last stocks until 2002 [Chicago Tribune May 22, 1999]. There is a fear that a rogue state like North Korea may also have a stockpile of smallpox virus. We have a unit on germ warfare later in this course.)



Typhus is caused by 'bacteria-like microorganisms' that are 'obligate parasites' of lice. Humans thus become an accidental host for the disease when exposed to the lice. The louse (singular for lice) epidemic form of typhus has been one of the greatest killers of mankind, especially in the aftermath of war when it attacks the cold and hungry.

This disease has disabled armies and devastated civilians. You might enjoy the classic reader on this disease, Zinsser's Rats, Lice, and History. (My note: Zinsser, a legend in microbiology was also a witty writer. He said that "Napoleon's retreat from Moscow was started by a louse!"

Typhus fever was introduced into the Americas by the Spanish explorers where it ravished the Indian population and even now persists in endemic form in Mexico, Central, and South America. There were major outbreaks in World War I, World War II and 1951 in Korea. It is always out there, but makes an appearance when social upheaval leave people cold, filthy and hungry.

Typhus is a disease of crowding and dirt, particularly likely to occur in conditions where people are crowded together and lacking means of ensuring bodily cleanliness. This disease clearly illustrates the importance of public health and social justice in the eradication of disease.

Transmission is by the bite of the louse. Typhus is always there among lice, ready to recur whenever public health measures are lax. Vaccination provides an effective, long-lasting immunity and is necessary for military personnel. Prevention also requires cleanliness and louse killing measures. This is a classic zoonotic disease that, unlike smallpox, remains endemic in the louse and lurks ready to appear in conditions of poverty and social disintegration.



This is a recent disease, with its history largely confined to the last two hundred years. The causative organism is a Spirillaceae bacterium. Cholera was originally confined to India early in the nineteenth century. It spread rapidly in association with two patterns of population movement: the movement of Islamic pilgrims to and from Mecca and to various parts of the world, and the deployment of British military throughout the Empire. Cholera, in fact continues to spread, arriving in new parts of the world, often accompanying industrialization--a process that has continued even past 1960.

Humans are the only known host for the cholera vibrio. The disease must be maintained by an unbroken chain of mild or subclinical infections. The true home on endemic cholera is the Yangtze River Valley in China and the delta of the Ganges in India.

Cholera remains endemic in Asia, the Middle East, Africa. and the Gulf Coast of the United States. It is spread by water, seafood, and other foodstuffs contaminated with the excrement of infected persons. The composer Tchaikovsky is said to have died of it in 1893 after drinking unboiled tap water in Russia.


My note: Epidemics of cholera and other water borne diseases in Chicago led to the engineered reversal of the Chicago River at the turn of the century with the opening of the Sanitary and Ship Canal. This ended forever contamination of Lake Michigan drinking water by Chicago sewage which heretofore had been emptied into the lake.

Earlier, in 1885, six inches of rain in less than a day caused such contamination of the Lake Michigan water supply that deadly outbreaks of typhoid fever, cholera, dysentery and other water borne diseases devastated the city. Twelve percent of the Chicago population died! Another disease epidemic hit Chicago in 1890; it lasted for two years, giving Chicago one of the highest death rates in the world.

Daniel Burnham (who said 'make no small plans') was determined to have pure water for the Columbian Exposition of 1894 on Chicago's South Side. He directed construction of a temporary 101 mile gravity pipeline from a freshwater spring at Waukesha, Wisconsin to the Exposition. Not one person became ill. A testament to what modern engineering could do for public health.


People living in areas endemic with cholera acquire a natural immunity. Vaccination, however, provides only incomplete immunity. Treatment of cholera is largely supportive for dehydration. Water and sewage treatment along with effective sanitation remains the best defense against cholera.

My note: Cholera vibrio microorganisms overcome a daunting ordeal to infect people. First, they face stomach acids which decimates them. Only one in a million microorganisms survives. They compete with other bacteria, long adapted for survival in the human digestive system and they may kill or suppress invaders.

It is called vibrio because it has a propeller-like tail to propel it and a way to sense crevices in the lining of the small intestine. There it releases a toxin that causes biochemical changes in the intestinal cells releasing a flood of water and electrolytes. This new induced environment favors the cholera organism over other intestinal organisms. The floods of fluids wash the competition away. Within hours, diarrheal discharges contain more than 100 billion cholera organisms per liter. How's that for overwhelming the competition?



Measles appears to have come from rinderpest in cattle or distemper in dogs. This viral disease in modern in origin and attacks all available human hosts resulting in death or a life-long immunity. Humans are the only host for measles. It must be passed on, person to person; there is no intermediate host or means of survival on inanimate objects.

It is perhaps the most infectious of all the specific human diseases. People of all races and all ages are infected readily by even a casual exposure. In most industrialized countries, measles is a childhood disease because most adults are immune as a result of a previous childhood infection.

A population of at least 230,000 to 500,000 persons is needed to sustain the disease. In smaller populations as on islands or among the Eskimo, the disease will infect every susceptible person and then lacking other persons, the disease will 'burn itself out' and disappear.

When measles first appeared amongst aboriginal populations, it swept through them with a violence that was unknown among more Old World nations in which the disease had been endemic for centuries. Such was the great epidemic in the Fiji Islands in 1874, and those which occurred in the South Sea Islands and among Native American Indians in the Americas.

In isolated communities, everyone may become immune by exposure to infection and the virus disappears. This results in a new generation which is universally susceptible to the disease. Note well: measles is a modern disease whose origin seems to be related to the growth of human population and its tendency to settle into dense aggregates or widespread and efficient trade networks. It is a disease of civilization, a classic 'crowd' epidemic disease which requires large numbers of people to persist. Although the virus comes from dogs or cows, the critical mutation only occurred just after the emergence of urban centers.

Treatment is symptomatic. An effective vaccine is available.

..... CJ '99


Cartwright, F. Disease and History. New York: Dorset, 1972.

Cohen, M. Health and the Rise of Civilization. New Haven: Yale University Press, 1989.

Ewald, P. Evolution of Infectious Disease. New York: Oxford University Press, 1994.

Garrett, L. The Coming Plague. New York: Farrar, Straus and Giroux, 1994.

Kipple, K., ed Plague, Pox & Pestilence. New York: Barnes and Noble Books, 1997.

Merck Manual 16th ed., Rahway, Merck Laboratories, 1992.

Smith et al Zinsser Microbiology. New York: Appleton Century Crofts Inc., 1960.

Wills, C. Yellow Fever Black Goddess Reading: Helix Books Addison-Wesley, 1996.