Infectious causation of some acute diseases was recognized early in the 1800s as a result of conspicuous chains of transmission. The infectious nature of smallpox, measles, and chicken pox was recognized by medical experts and the general public decades before the microscope led Koch, Pasteur, and other early microbe hunters to the first cause-and-effect linkage of bacteria with disease during the 1870s and 1880s. Before the identification of microbes, the concept of infection was less tangible, and the distinction between infection (the invasion and growth of a disease-causing entity in the body) and contagion (the spread from one body to another) was often blurred. But infection was invoked to refer to diseases caused by something that grew inside people and could be transmitted to others to continue the process.
No special training was needed to witness the chains of transmission of many respiratory tract diseases, only a moderately observant eye. Parents noticing chicken pox in their child’s playmate would see chicken pox in their own child within a couple of weeks, and then chicken pox in another playmate within a couple of weeks after that. They wouldn’t have to be a Pasteur to realize that something was growing inside the bodies of their kids and being transferred from child to child. Though the viruses that caused these diseases were not seen with a microscope until the middle of the twentieth century, a half century after Pasteur, the medical texts before Pasteur’s time already had the causation and transmission right for these diseases. These texts had it wrong, however, for every other category of infectious disease.
At the end of the nineteenth century, after the microbe hunters of Pasteur’s time had observed some of the bacteria that cause diarrheal diseases, the writers of medical texts were still arguing about whether the bacteria caused the diarrheal diseases or were just innocent bystanders. The confusion can be seen in the standard American medical text of the 1880s, A Dictionary of Medicine. “Asiatic cholera” to be infectious, but transmission rather than infection was emphasized. “Simple cholera” was considered nontransmissible. This distinction probably arose because Snow’s work left little doubt that epidemic cholera—which could be traced to south Asia and was therefore termed Asiatic cholera—was waterborne. Snow deduced that the cause of the disease must be some organism that could multiply inside people, be shed in the feces, and be transmitted in fecally contaminated water and food. No one had done similar work on “simple cholera,” which probably included what are now recognized as several different diseases characterized by watery diarrhea. So those who favored noninfectious explanations for diarrheal disease could justify relinquishing only the epidemic cholera. The Dictionary noted the presence of the typhoid bacterium in typhoid patients but attributed typhoid fever to transmissible “typhoid poison” rather than transmission of the organisms themselves.
Why were acute diarrheal diseases recognized so much later than the acute diseases of the respiratory tract? A simple exercise can reveal part of the answer. When you go to work, school, or wherever you mingle, study the people next to you. Look and listen for signs of respiratory tract infection. If someone is infected with, say, one of the rhinoviruses that cause the common cold, you will have a good chance of detecting it. You may see or hear them sneeze. If they talk, you may hear the telltale muffling of nasal congestion. Then look and listen for signs of diarrheal disease. It’s not so easy. Other avenues of information are also restricted. Say you see a
not-so-close acquaintance who looks a little peaked and ask, “Feeling all right?” You might get the response, “Oh, I’ve got a cold.” But how many times have you heard “Oh, I’ve got diarrhea”? People prefer not to broadcast information about such matters. It tends to be reserved to very close friends and family. It is not surprising that infectious causation of diarrheal diseases, being reinforced by less evidence from common experience, was generally accepted later than infectious causation of acute respiratory tract diseases.
Infectious causation of diarrhea was probably also more difficult to recognize than infectious causation of respiratory tract diseases like chicken pox because many people who are infected with diarrheal pathogens show no symptoms at all. Diarrheal diseases also can be transmitted over long distances—for example, by fecally contaminated water. So if one is assessing whether diarrheal diseases are a result of something growing in a person and being transmitted to other people, one will often find seemingly contradictory information. If one chooses as a study subject someone who acquired an infection from an asymptomatic person or through the water supply, no direct contact with diarrhea may be apparent, no matter how careful and thorough the study.
Acceptance of infectious causation of vector-borne diseases was delayed for similar reasons. If a disease is transmitted by a mosquito, an observer might track down every contact of a sick person without turning up another person who has the disease. That is just what happened in a study of yellow fever in Barcelona. In 1822 the French government was still trying to learn the lessons of the Haitian revolution that had occurred two decades before. The Haitians were successful in large part because yellow fever killed thousands of French soldiers while leaving Haitians relatively unscathed. Wanting to figure out the problem before their next clash with yellow fever, the French government sent Nicholas Chervin to Barcelona to determine whether yellow fever was contagious. Chervin’s careful studies documented that many people who came down with yellow fever had no contact with anyone else who had yellow fever. He concluded, logically enough, that yellow fever was not transmissible between people. He did not consider other possibilities for long-distance transmission. If one does not entertain the full range of possibilities, one may miss the right answer. Infectious causation of vector-borne diseases remains obscure if one does not consider the possibility of vector-borne transmission. The Cuban epidemiologist Carlos Finlay did so six decades later, correctly implicating Aedes mosquitoes as the vector for yellow fever. Still, the idea of infectious causation of yellow fever was often rejected out of hand. In 1898, for example, an official bulletin of the U.S. Marine Hospital Service stated that “one has not to contend with an organism or germ which may be taken into the body with food or drinks but with an almost inexplicable poison so insidious in its approach and entrance that no trace is left behind.” When one isn’t on the right track, problems do seem inexplicable. Acceptance of yellow fever’s infectious cause finally occurred a few years later, after Walter Reed’s commission published the results of its experimental transmission of yellow fever to humans.
“Oh, I’ve got diarrhea” would be an unusual response to a casual “How are you?” but “Oh, I’ve got a venereal disease” may never have been given. That kind of information is withheld even from close friends and family—sometimes especially from close friends and family. One might try looking and listening for evidence of sexually transmitted infections among casual acquaintances, but as with diarrheal diseases, telltale signs are lacking. Sexual transmission also introduces a novel source of crypticity. Because few people have sex in public and most do not go around broadcasting their sexual activities, knowledge about who has had potentially transmissible contact with whom is lacking.
Not surprisingly, the infectious causation of sexually transmitted diseases was also recognized later than that of respiratory diseases. The Dictionary of Medicine attributed gonorrhea to causes such as too much physical disturbance of the genitalia by too much sex or masturbation. The presence of Neisseria gonorrheal which had been discovered five years earlier, was mentioned but was interpreted as possibly being that of an innocent bystander. Syphilis was referred to as “a specific contagious, noninfectious disease; communicable by contact of the poison with a breach of surface, or by hereditary transmission.”
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